To un-jam your garbage disposer you may be able to use a disposer wrench. The disposer wrench is essentially a 1/4 inch Allen wrench which fits into the bottom center, underneath the disposer. Insert the wrench and wiggle the grinding plate back and forth to free up the jam. If the grinding plate is very stiff and won't loosen up, the disposer will probably have to be replaced.
If you hear a grinding noise when you wiggle the plate back and forth there might be something caught in the disposer. Common items that get caught are coins, earrings, soda can tops, twist ties, toothpicks and more. A strong flashlight can be used to inspect the inside of the disposer. The jam usually happens at the wall of the disposer where the grinding plate meets it.