@dlowan,
What's going on with it? Won't turn on? Turns on but doesn't spin? Drain backs up?
If it turns on but doesn't spin, then it's probably jammed. Turn it off (preferably at the breaker). There should be an allen wrench (the end looks like a hexagon) that fits in the bottom. Try turning anti-clockwise to release the jam, then reach in and see if you can find the debris that jammed it. (The reaching in part is why you turn it off at the breaker.)
If the drain backs up, you should be able to find one of these at your local hardware shop:
http://www.amazon.com/Cobra-Products-331-Cleaning-Attachment/dp/B000KKRQUM/ref=sr_1_16?s=kitchen-bath&ie=UTF8&qid=1334451443&sr=1-16
Under the sink, you disconnect the drain, then you stick this thing in it, attach a hose, and turn on the water. The bladder inflates, sealing the drain, and forces water down the drain.
These come in various sizes, so you can get 'em that fit kitchen sinks all the way up to ones you stick in the toilet.