Pls.Try以清除以下位置的日志文件，然后重新启动您的SQL Server： -

C：\\ Documents和Settings \\你的电脑用户\\本地设置\\应用数据\\微软\\ Microsoft SQL Server数据\\ SQLEX $ P $干燥综合征

I faced this error sometimes or sometimes not (sometimes app run properly),but now it is giving me error again and again. I am running my application on local host.Following are the error details. I am not sure why I am facing this error, I think I am doing something wrong. Due to this error I am stuck at my further working on my project.

A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)

Web.config:

<?xml version="1.0" encoding="utf-8"?>


<configuration>
  <configSections>

    <section name="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />

  <sectionGroup name="dotNetOpenAuth" type="DotNetOpenAuth.Configuration.DotNetOpenAuthSection, DotNetOpenAuth.Core">
            <section name="messaging" type="DotNetOpenAuth.Configuration.MessagingElement, DotNetOpenAuth.Core" requirePermission="false" allowLocation="true" />
            <section name="reporting" type="DotNetOpenAuth.Configuration.ReportingElement, DotNetOpenAuth.Core" requirePermission="false" allowLocation="true" />
        <section name="openid" type="DotNetOpenAuth.Configuration.OpenIdElement, DotNetOpenAuth.OpenId" requirePermission="false" allowLocation="true" /><section name="oauth" type="DotNetOpenAuth.Configuration.OAuthElement, DotNetOpenAuth.OAuth" requirePermission="false" allowLocation="true" /></sectionGroup></configSections>
  <connectionStrings>
    <add name="DefaultConnection" connectionString="Data Source=(LocalDb)\v11.0;Initial Catalog=mydb;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|\mydb.mdf" providerName="System.Data.SqlClient" />
  </connectionStrings>
  <appSettings>
    <add key="webpages:Version" value="3.0.0.0" /> <!--i have made new version from 2.0.0.0 to 3-->
    <add key="webpages:Enabled" value="false" />
    <add key="PreserveLoginUrl" value="true" />
    <add key="ClientValidationEnabled" value="true" />
    <add key="UnobtrusiveJavaScriptEnabled" value="true" />
  </appSettings>
  <system.web>
    <compilation debug="true" targetFramework="4.5" />
    <httpRuntime targetFramework="4.5" />
    <authentication mode="Forms">
      <forms loginUrl="~/Account/Login" timeout="2880" />
    </authentication>
    <pages>
      <namespaces>
        <add namespace="System.Web.Helpers" />
        <add namespace="System.Web.Mvc" />
        <add namespace="System.Web.Mvc.Ajax" />
        <add namespace="System.Web.Mvc.Html" />
        <add namespace="System.Web.Optimization" />
        <add namespace="System.Web.Routing" />
        <add namespace="System.Web.WebPages" />
      </namespaces>
    </pages>
  </system.web>
  <system.webServer>
    <validation validateIntegratedModeConfiguration="false" />
    <handlers>
      <remove name="ExtensionlessUrlHandler-ISAPI-4.0_32bit" />
      <remove name="ExtensionlessUrlHandler-ISAPI-4.0_64bit" />
      <remove name="ExtensionlessUrlHandler-Integrated-4.0" />
      <add name="ExtensionlessUrlHandler-ISAPI-4.0_32bit" path="*." verb="GET,HEAD,POST,DEBUG,PUT,DELETE,PATCH,OPTIONS" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" preCondition="classicMode,runtimeVersionv4.0,bitness32" responseBufferLimit="0" />
      <add name="ExtensionlessUrlHandler-ISAPI-4.0_64bit" path="*." verb="GET,HEAD,POST,DEBUG,PUT,DELETE,PATCH,OPTIONS" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" preCondition="classicMode,runtimeVersionv4.0,bitness64" responseBufferLimit="0" />
      <add name="ExtensionlessUrlHandler-Integrated-4.0" path="*." verb="GET,HEAD,POST,DEBUG,PUT,DELETE,PATCH,OPTIONS" type="System.Web.Handlers.TransferRequestHandler" preCondition="integratedMode,runtimeVersionv4.0" />
    </handlers>
  </system.webServer>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="DotNetOpenAuth.Core" publicKeyToken="2780ccd10d57b246" />
        <bindingRedirect oldVersion="0.0.0.0-4.3.0.0" newVersion="4.3.0.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="DotNetOpenAuth.AspNet" publicKeyToken="2780ccd10d57b246" />
        <bindingRedirect oldVersion="0.0.0.0-4.3.0.0" newVersion="4.3.0.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="System.Web.Helpers" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="1.0.0.0-2.0.0.0" newVersion="3.0.0.0" /><!--i have made new version from 2.0.0.0 to 3-->
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="System.Web.Mvc" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="0.0.0.0-5.2.2.0" newVersion="5.2.2.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="System.Web.WebPages" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="1.0.0.0-2.0.0.0" newVersion="2.0.0.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="WebGrease" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="0.0.0.0-1.6.5135.21930" newVersion="1.6.5135.21930" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="System" publicKeyToken="b77a5c561934e089" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-4.0.0.0" newVersion="4.0.0.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="Newtonsoft.Json" publicKeyToken="30ad4fe6b2a6aeed" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-6.0.0.0" newVersion="6.0.0.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="System.Net.Http" publicKeyToken="b03f5f7f11d50a3a" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-4.0.0.0" newVersion="4.0.0.0" />
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="Antlr3.Runtime" publicKeyToken="eb42632606e9261f" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-3.5.0.2" newVersion="3.5.0.2" />
      </dependentAssembly>
    </assemblyBinding>
   <!--This prevents the Windows Event Log from frequently logging that HMAC1 is being used (when the other party needs it). <legacyHMACWarning enabled="0" /> When targeting ASP.NET MVC 3, this assemblyBinding makes MVC 1 and 2 references relink
             to MVC 3 so libraries such as DotNetOpenAuth that compile against MVC 1 will work with it.-->
    </runtime>
  <entityFramework>
    <defaultConnectionFactory type="System.Data.Entity.Infrastructure.LocalDbConnectionFactory, EntityFramework">
      <parameters>
        <parameter value="v11.0" />
      </parameters>
    </defaultConnectionFactory>
    <providers>
      <provider invariantName="System.Data.SqlClient" type="System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer" />
    </providers>
  </entityFramework>
<system.net>
        <defaultProxy enabled="true" />
        <settings>
             <!--This setting causes .NET to check certificate revocation lists (CRL) 
                 before trusting HTTPS certificates.  But this setting tends to not 
                 be allowed in shared hosting environments.--> 
            <servicePointManager checkCertificateRevocationList="true" />
        </settings>
    </system.net><dotNetOpenAuth>
        <messaging>
            <untrustedWebRequest>
                <whitelistHosts>
                     <!--Uncomment to enable communication with localhost (should generally not activate in production!)--> 
                    <add name="localhost" />
                </whitelistHosts>
            </untrustedWebRequest>
        </messaging>
         <!--Allow DotNetOpenAuth to publish usage statistics to library authors to improve the library.--> 
        <!--<reporting enabled="true" />-->
     <!--This is an optional configuration section where aspects of dotnetopenauth can be customized.  For a complete set of configuration options see http://www.dotnetopenauth.net/developers/code-snippets/configuration-options/--> <openid>
        <relyingParty>
                <security requireSsl="false">
                     Uncomment the trustedProviders tag if your relying party should only accept positive assertions from a closed set of OpenID Providers. 
                    <trustedProviders rejectAssertionsFromUntrustedProviders="true">
                        <add endpoint="https://www.google.com/accounts/o8/ud" />
                    </trustedProviders>
                </security>
                <behaviors>
                     <!--The following OPTIONAL behavior allows RPs to use SREG only, but be compatible
                         with OPs that use Attribute Exchange (in various formats).--> 
                    <add type="DotNetOpenAuth.OpenId.RelyingParty.Behaviors.AXFetchAsSregTransform, DotNetOpenAuth.OpenId.RelyingParty" />
                </behaviors>
            </relyingParty></openid></dotNetOpenAuth><uri>
         <!--The uri section is necessary to turn on .NET 3.5 support for IDN (international domain names),
             which is necessary for OpenID urls with unicode characters in the domain/host name.
             It is also required to put the Uri class into RFC 3986 escaping mode, which OpenID and OAuth require.--> 
        <idn enabled="All" />
        <iriParsing enabled="true" />
    </uri></configuration>

Pls.Try to clear log files from following location and restart your sql server :-

C:\Documents and Settings\Your pc-user\Local Settings\Application Data\Microsoft\Microsoft SQL Server Data\SQLEXPRESS