Only one thumbnail for all photos
When I had this problem it was caused by a corrupt SD card, so you should check that out soon: the longer you leave it, the more likely it'll corrupt a file you don't have a backup of.

For now, try clearing the cache of any app that shows the problem (from Settings → Apps). If that doesn't help, clear the data of the Media storage app, from the same screen, and then reboot the phone. The Media storage app is responsible for indexing your photos and music and letting apps search them, so clearing its data often helps fix problems with the photo index, such as broken thumbnails.