Check if you have your device connected to a PC via USB connection in Mass storage mode. If connected in Mass storage mode, the photo folders located on the mass storage drive or memory card cannot be viewed while the device is connected to the PC. The folders will show zero items in this situation.
Unplug the USB connection to be able to view the images again in the Photos application.
The issue occurs only when connecting to PC by using the Mass storage USB mode. If you want to continue viewing the photo folders while connected to a PC, use the PC Suite or Nokia Suite connection mode instead.