Server Explorer is not working.
This error occurs while opening a project with a database reference. Visual Database Tools checks to make sure that Server Explorer contains the designated database connection. For some reason, the database connection is missing. There are many possible reasons for this error. You can try any of the following remedial actions:
Repeat the operation.
Close the solution and try again.
Restart Visual Studio and try again.
Recover a previous version of the project from source control.