These motors have capacitors that give them a jump start to get then going from standing still. If the capacitor gets weak or dies, the motor cannot develop enough torque to get the belt running. You can check if this is the case by starting the treadmill and then carefully try to drive the belt with your feet while standing on the belt. Once the belt starts moving, the capacitor is no longer needed, so the belt/motor should keep going. If this test doesn't work, and there is nothing jammed within the treadmill to keep it from running, then most likely your motor is bad.
Hope this helps.
fudd, June 2009