Waste disposals will often stop working mainly when they get a little seized up. This can happen if you haven't used it for a while or if you don't flush enough water through when using it. You should run the tap before using it and when putting food through, then, for around 15 seconds afterwards (uses quite a bit of water so if you want to save water put your veggies in the bin!)
Most problems I find are caussed by food blockages having been inadequately flushed through with water.

Things to try

1 If it's not working and you hear a fairly quiet buzzing/humming sound when you turn it on this is the motor straining to rotate.

Turn off the power. Underneath in the middle of it you should find a hexagonal hole for an allen key (which will have come supplied) if you've lost it you'll need to buy a suitable sized replacement
Fit the allen key in the hole and try to turn it. You'll find quite stiff at first.

Having freed the waste disposal you should be able to spin it freely with the allen key. Remove the key and try tuning it on.

2 You will also find underneath a small round reset button.

If you keep trying to turn on the unit when its jammed (and hear the buzzing /humming sound) eventually the motor will over heat an a safety cut-out will cut the power and the buzzing will stop. Free the unit up with the allen key as above.Then press the little re-set button.

3 If the reset doesn't work the unit may have blow the fuse in the switch. Try changing this. (see changing fuses) If it's been installed properly you should find a fused switch close by.

Open up the fuse holder and test the fuse. Change it if necessary.