Your disposal is stuck. Sounds like something either fell in it, or perhaps a bone got inadvertantly tossed in. Do you own an allen wrench set? The garbage disposal did originally come with one, but it usually gets lost. The one that came with it will look like two hockey stick stuck together. In any event, you need one for the fix. There is a hole on the bottom of the disposal, directly in the center. Find the correct size allen wrench and place in this hole. Twist clockwise and counter clockwise until it is free. Once this is done, you most likely will need to hit the reset button on the bottom. Good luck!

WoW, i had 100% same problem and 100% same solution, all done by an allen rench from work. Thank you.