PLEXTOR DVD EXTERNAL DRIVES

I have a Plextor PX-716UF. This external drive has worked PERFECTLY for 4 years. I really don't like my HP DVD drive that came with my PC, thats why I purchased this external drive. All of a sudden tonight, it won't burn any discs, it won't even play a movie. You put in the discs, and it does nothing.
I also have a Plextor PX-708UF, as a back up. I dragged it out of the closet...same thing.
Thought it might be the Firewire cord, so I tried two different ones....Ok so I tried the 2.0 USB....I tried 3 different cords, and every 2.0 on my HP....same thing..nothing. Now my Plextor shows up on my PC in hardware, so it knows its there. The ONLY thing I have done in the last 2 weeks is upgrade more ram. Any ideas..on why this is happening. If my laptop wasn't Vista, I would try it on it. BTW I have Windows XP.
thanks for any ideas.