The last time that happened to my pump, it was because the shaft, where it comes out of the front of the motor and connects to the impeller in the pump, had broken in half. This can happen if that extension piece is plastic, and something gets hung in the impeller and stops it from turning, although the motor is still trying to turn. The plastic piece can just twist in half, so the motor runs but the impeller doesn't turn.

Just a different idea, although Poconos knows WAY more about motors than I do, so try his suggestions first!

Janet