I imagine that this is because the files don't exist. Have you created the project on a network drive somewhere? Have you renamed or moved the folder after adding it to the solution?
Check the paths are right, then double check. Also check your version control to ensure someone hasn't wiped out your changes and left the .sln file intact.
You can edit the .sln directly I suspect to double-check the exact path it's looking for. Verify that path exists too.
Make sure the project you added was created in the same version of Visual Studio too.