I faced this problem... while I restarted my solution. (Not sure what I did wrong... may be my VPN was not connected and I was not able to connect to TFS) then it took my solution offline and then all this problem started.
Unable to read the project file. The imported project file could not be loaded. Root element is missing.
Don't reinstall Visual Studio or even .NET framework.
What you have to do is delete the .user file for that project and it will work.