One of the three tabs the propeller uses to lock in place is probably broken off and lodged into the motor housing. Just remove the propeller and try to remove the broken plastic tab and it should spin freely again. The other option is the motor shaft is bent, or the cylindrical housing is bent disallowing the motor to spin freely. Mine had the tab broken upon impact. good luck to anyone with the problem. The motors are not sold individually [yet] so the option is to replace the arm, between $45 and $80 depending on which arm and the supplier. The process requires soldering the new arm in place.