Could be one of two things:-
(1) The triac that controls the motor ciircuit has failed short circuit, and basically is continuously running the motor.
(2) A component on the sound circuitry has failed.
Or, an easy problem to fix is:-
(3) Gearbox on the motor is making a lot of noise, which is then being picked up the mic and triggering the motor in an endless loop
Assuming that you haven't got any Sound / continuous motor mode selection switch set to "continuous" - (I have to ask

you'd be surprised how many people overlook this!!).
If it's (1) or (2) then unless your good with a soldering iron then you are stuck. Sorry