Welcome to Android Central! First, install
SD Insight to see if the card is genuine or counterfeit. Counterfeit cards are unfortunately not uncommon, and are typically much smaller capacity than they say they are. When you try to save more than they can actually store, then the files can get corrupt.
If SD Insight says the card is valid, then go to
Settings>Storage, Unmount the card, and then remove it. Insert it into your computer and see if the computer can read those files using the computer's file explorer program. While you're at this stage, back up any important data from the card to the computer's hard drive, in case the card is on the verge of failing.
If the computer can't read the file, the file or the card might be corrupt. You could try running chkdsk (
http://forums.androidcentral.com/am...de-using-chkdsk-fix-corrupted-sd-card.html)to look for bad sectors--but if it finds bad sectors and repairs them, you might lose that photo altogether (if it was in one of those bad sectors). So beforehand, you could also consider using a file recovery program:
http://forums.androidcentral.com/am...-s/500142-guide-recovering-deleted-files.html
Good luck!