We were recently faced with the error message The project file ' ' has been renamed or is no longer in the solution in Visual Studio 2005. The problem is that from this message you have no idea what is actually the matter. We finally figured out that this happens when a Web Project contains references to assemblies or projects it can't find. Here's how you solve this:

1. Right click the Web project and select Property Pages.
2. A window will open which lists all the references, either to the bin-folder, GAC or other projects in the solution.
3. Remove those that show (unavailable) behind it.
4. Chances are that now you can't build because the reference is not there. Simply add the reference again and you should be OK.