Did they disappear from your SD Card, internal memory, Cloud service? If you try to browse the folder where they were through the File Manager, can you see them? If so, then some setting in the Gallery is hiding them.
If it's an SD Card, I'd try to read it on a computer and see if they appear there.
Another couple of options: The pictures are set as "hidden" and can only viewed if you enable that setting, or someone else just went ahead and deleted them.