I had the same problem with one specific solution and Visual Studio 2013. It seems that the problem can be solved by simply deleting the .suo file (you can find it in the same directory as the .sln file).
Please note that your user settings for the solution are stored in the .suo file, so after deleting it you will lose the previously opened tabs, the choice to start the project and some other things, but nothing really important. Visual Studio will recreate this file the next time the solution is opened.
Update: In the case of Visual Studio 2015/2017, you need to delete the .vs folder .vs .
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