your gears prob fell to pieces, as many are made of plastic. Happened to me, I had to remove position sensors/cam and rebuild it for now w/o them...still works, just have to stop pressing the sw before it goes too far or it'll strip the main drive screw. Yes, you can twist the knurled knob by hand w/the motor removed. You'll see when you have it apart, there used to be nice pics posted here...
