First, you should try reconnecting the file in PSE. File->Reconnect.
You would use the backup only if that doesn't work at all.
However, if you made the backup using the built-in PSE backup command, then there really is no easy way to find the photo, as PSE renames the photos in its backup. So, I think you have to scroll through the photos in the backup until you find it visually. Now that may not be as terrible as it sounds, as I believe the photos still are ordered chronologically, so if you know the photo was taken in December 2008, that shouldn't be too hard to find.