You'll need to go through the possibilities small steps at a time. You need to make sure the motor and gearbox both turn freely by hand, sometimes the bearings can seize,magnets fall off inside the motor and any number of other possibilities.
You might as well start by stripping and servicing the motor and gearbox, that way you have a good "base line" to work from.
Then power up the motor and take it from there.
The keyswitch connections, pod connections and control box (plus fuses) all need checking to eliminate any other possibilites.
Should keep you busy for a weekend