Solved: The (Visual Studio) project file * has been moved, renamed or is not on your computer.

I renamed my Visual Studio 2010 (VS2010) Solution (.sln) and Project (.vcxproj) files today.  When I opened the Solution, it could not find the Project file, thinking it was still in the old location.  To solve this, I had to exit VS2010, delete the project’s .suo file, restart VS2010, open the Solution, and then re-add the Project file.

I found this answer here, in reference to VS2005.