Will either be the start capacitor (in the box on top of the motor) or something binding the impeller or potentially both.

There is a fan cowling on the rear end of the pump try this;
- Turn the power off
- Remove the cowling
- See if the fan allows you to spin freely/easily the pump
BE SURE nothing could become entangled in the fan
- Turn pump back on
- If it starts, probably something on the impeller side
- If it doesn't but will start with a little turn of the fan probably the capacitor. Get a new one from Jaycar or RS components

Remember to put the cowling back on safely.