尝试工具> Android>使用Gradle文件同步项目.如果在完成该过程后一切都没有改变，请关闭并重新打开IDE.

Try Tools > Android > Sync Project with Gradle Files. If nothing changes when that process seems complete, close and reopen the IDE.

如果仍然不能解决问题，请通过文件">关闭项目"关闭项目，使用开发计算机的文件管理器(例如，Ubuntu中的Nautilus，OS X中的Finder)进入项目目录，然后删除.iml 文件.然后，从您选择较早关闭项目时出现的欢迎对话框中，使用导入非Android Studio项目"导入项目.

If that still does not help, close the project via File > Close Project, go into the project directory using your development machine's file manager (e.g., Nautilus in Ubuntu, Finder in OS X), and delete the .iml file. Then, import the project using "Import Non-Android Studio Project" from the welcome dialog that appeared when you chose Close Project earlier.