If you don't know this design, There is a common belt between the motor and both wheels. Many times this bely will dry out, fail, or slip.
The next thing is to check the motor itself. This turns on from an under playfiled relay board. Is this relay firing in coil test.
If not firing it's a wire, voltage, or bad relay on this board or power to this board.
If this relay fires then check the voltage at the motor with the relay fired. If the motor is getting voltage and not turning then it's your motor.
Usually it's the belt, but I have seen these relays bad, and I have also seen these motors bad.