Thumbnails are greyed out on iPad 2, however the photo is still there when I open the photo. How do I fix this?

Hi. I have the ipad2 with iOS 6.1.3. The photos in my photo library have blank thumbnails when I open the album, looks like a black grid with white lines instead of photos. The photos open and view fine when I view an individual photo or slideshow. How do I get the thumbnails to show?

Thanks!