Hey there, patmollman.
Unfortunately, recovering data from a failed NAND flash device is often pretty much impossible, so as long as you still have access to those files, I'd strongly recommend that you back them up, just to be on the safe side. Keeping important information on a flash drive or an SD card without a backup is not a really good idea.
After that you could try reformatting the flash drive, to see if that fixes things up. If it doesn't and if it's still under warranty, I'd recommend that you try to RMA it.
Hope that helps.
Boogieman_WD