这可能是服务器本身，S2S VPN或On-Prem的问题。

This could be an issue with the server itself, the S2S VPN, or an issue from On-Prem. 

第一步是缩小问题的来源。最简单的方法是查看是否可以连接到同一VNET中的其他服务器，或者查看是否可以在发生这种情况时连接到备份服务器的公共IP。 

The first step is to narrow down where the issue is coming from. The easiest way to do that is to see if you can connect to a different server in the same VNET, or see if you can connect to backup server's public IP while this is happening. 

如果您能够从公共IP而不是VPN访问您的VM，则VPN存在问题。如果您无法通过任何一种方法访问服务器，则可能是服务器出现问题。 

If you are able to reach your VM from the public IP but not the VPN, it is an issue with the VPN. If you are not able to reach the server via either method, it is likely an issue with the server.