How can I recover thousands of scanned photos from WD 500 gig external drive?

After spending weeks scanning THOUSANDS of family photos from 6 generations and backing them up to my WD external hard drive, the only thing left are a bunch of folders with absolutely nothing in them!  I did not delete them, but they are not there, please help me recover these memories, thank you

The files might be corrupted, try finding a data recovery program for Mac and scan the hard drive.