The SD card could be corrupt, defective, or counterfeit. Install SD Insight to see if the card is genuine or counterfeit. Counterfeit cards are programmed to report more storage than they actually have, and if you try saving more than the card can actually hold, then files can start getting corrupt.
If it's a valid card, then go to Settings>Storage, Unmount the card, remove it, and insert it into your computer. Can the computer read these files? Backup as much as you can right now, then run chkdsk to look for bad sectors: http://forums.androidcentral.com/amb...d-sd-card.html. If it finds bad sectors, and the photos aren't crucial to save, then go ahead and allow chkdsk to try repairing those sectors. If the photos are really precious, then you'll have to try recovering them using 3rd party software: http://forums.androidcentral.com/amb...ted-files.html