First thing is : Don't Panic
You can tell if the motor is seized by gently rotating the rear drive wheel "in reverse" - any problems with the gearbox/motor structure will be readily obvious by the strange noises you'll hear or lack of movement. You could also directly connect the motor to a battery if you wanted to hear it run - disconnect the motor leads from the control box to do this.
Then it is a process of elimination, check all the connections - especially the red wires that connect to the rear of the pod . If it is running the standard Sinclair keyswitch give it a good spray with WD40 and keep turning it on/off to free it all up. Also, take a look at the fuses in the control box.
If you are confident enough, you could take the working control box/pod off your "good" c5 and try it out on this "defective" one.
That way you'll be able to work out which components are in need of repair.
Good luck and let us know how you get on.
It still sounds like a bargain, even if you do find you have a worse case scenario of duff control box/pod.