Could be many things... If the wheel is stuck when the system is switched of and the battery is disconnected than a mosfet in the rearwheel has blown out. if the rearwheel is stuck when the system is off and the battery is connected a mosfet in the controller (in the battery package) could be blown out or the battery is below the charge capacity to start charging with a wall charger. It would lead to a B026 or a B820 or B821 error message on the BBI.