In my experience the file is still there you just need to find it. I open the project file in a text editor and then see what file it is (assuming there’s any text shown) and then put the real path in through the text editor.
Another way I’ve fixed it is to ignore the error and compile and see what object is missing. Locate it on disk. Close the project and then reopen it. When the Locate dialog appears I know where it’s at.
Dumb ways to resolve it but it’s worked for me the few times I’ve had it happen.