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.