You've got something caught in it. If you didn't throw it away, it probably came with a hex-key wrench. Make sure the thing is off, then look in the middle of the bottom of the thing. The wrench will fit into the hex hole. Move it back and forth until you can free up the motor. Take the wrench out, then turn it back on and see what happens. Lacking that wrench or an allen wrench that fits (you may not have enough room to get a good grip since these are usually smaller in length and offset), you can stick a broom handle down the drain and try to get it to move.