Problem retrieving copied photo files from USB flash disk

Hello,
I have been using SanDisk USB 4 GB cruzer drives to save photos from my camera and video files for over a year. I did this recently, the same as always and many of my photos cannot be retrieved. When I pull up the drive, the photos are there with a file name underneath, but a small "x" in the middle of the screen is all that can be retrieved. I feel like the files are there but for some reason cannot be pulled up. I took the drive to Walmart to try and retrieve them on a different computer and the same thing happened there. Any way to retrieve the photos?