For most disposers, a disposer wrench can generally handle most jams. A disposer wrench is a quarter-inch Allen wrench that fits into the bottom center of most disposers. If your disposer is jammed, insert the wrench and wiggle the grinding plate black and forth to loosen jammed particles.
If there's a grinding sound when you wiggle the plate, there could be something caught in the disposer. Common items to get caught are coins, earrings, soda can tops, twist ties, toothpicks and others. Use a flashlight to inspect the inside of the disposer.