What do you mean "nothing is working"? What did you try, what are the EXACT steps, and what happened, what was the EXACT error message?
The usual causes for "cannot be found."
1. You cataloged the photos from a camera or card without properly copying them to a library on your drive so when you disconnected the camera, the tumbs reman but the files are gone.
2. You inadvertently deleted the photos.
3 They are stored on a detachable or networked drive that has not been mounted.