The trimmer is either broken or clogged up with a combination of shavings and skin oil. If it is broken, there are no replacements available. If it is clogged, you can fix it by thoroughly cleaning it. Dip the blades of the trimmer into some rubbing alcohol or other mild solvent type liquid. Soak it for about one minute. Then take an old toothbrush and firmly scrub the teeth. Then turn it on and it should free it up. If it still doesn't move you can advance the blade manually to get it 'unstuck'. One of the blades is fix and the other moves back and forth. You should be able to see which one is supposed to move by sight. Take a small flat screwdriver or similar object and push the blade to each side once. Turn it on and it should move freely. Dip the trimmer blades back in the rubbing alcohol while it is running and let it run for about 15 seconds.
If still not working it is beyond repair. The easiest and least expensive way to get working again would be to purchase a new ;lower unit' which includes the trimmer. You will find at shaveroutlet.com. See link below