Another update of Visual Studio caused the same effect again. If you had to update this week, you might have been faced to the situation that your project cannot open again. If that is the case, the same solution can be applied to a previous situation already known to VS. As Viv mentioned, the following link resolves the issue:

http://stackoverflow1453.wordpress.com/2013/09/02/visual-studio-2012-error-no-exports-were-found-that-match-the-constraint-contract-name-how-to-solve/