It doesn't happen often, so when it does, I have to research the whole process all over again. Yesterday the garbage disposal started screeching. I immediately turned it off to see what the problem was, but couldn't see anything. When I turned it on again, it just hummed. Several years ago this happened and we called a plumber, who promptly turned a knob, dislodged a bunch of carrot peels, and got it working again. Before researching new garbage disposals, I figured I should research where that magical knob was located.
Note: If the unit is plugged in and you hear no sound at all, push the reset button.
It's at the bottom of the unit, in the center. Find a 1/4" allen wrench and wiggle it around. That didn't work at first, but since the YouTube video (found here) said if you hear a hum it's probably just jammed and not broken, I decided to investigate further. There was a piece of metal that didn't look like it belonged - originally I thought it was part of the disposal, but it didn't match the pictures I saw - so I tried harder to dislodge it. Success! Now our disposal is working again.
And now I need to buy a new 1/8" frosting tip!! That should be quite a bit less expensive than a new disposal.