Why does my PSP not register any UMD's at all?

My PSP 3000 is not "knowing" that any umd's (universal media disc, things like games, movies etc.) are in the device. This problem occurred first when I swapped the discs around to play a different game. I had just bought a new game and was trying it out. The previous game that I had used had been in the system for a while, about two to two and a half months. I had been ccompletely finishing it off. Can you help? Please?