My Time Machine has stopped working ...

I have been having real problems with my Time Machine. It was running very slow (taking hours!) then it stuck on 7MB out of 26MB. Today the computer can't even find the Time Machine. The TM still has a green light on it ... and I thought to 'forget' it and reset it up but Airport Utility cannot locate it now.

I'm in a real mess ... is my TM broken?