I'd try moving that swapped mp3 to another folder, first, and see if the save will load without it. If it loads to that point, cancel looking for the mp3, and see if it finishes loading.
If it loads without the mp3, then make the mp3 a wav, and put it back into the project.
If it fails, try it without other files, one at a time.
There's always the possibility that the save files are boned, regardless, but it's worth trying.