Damaged extension cord plugs should not be used. Typically, extension cords will develop cracks in the outer sheathing of the wire's insulation or plug problems will occur. A cord plug missing a ground prong is unacceptable and should be replaced.
To do this, take a pair of diagonal pliers and cut the old cord plug from the cord. Strip the cable sheathing back about 1 ½” TO 2" using a razor knife. Be careful not to cut too deeply. You don't want to cut into the wire insulation. Now strip the wire insulation back about ½” and expose the copper wire of each of the conductors. The cord is now prepped for the new cord plug.