The first thing to know is that you should only apply power in short bursts with cooling time in between when the motor doesn't spin up normally. This is to prevent damage to the motor windings.
Failure to start up is usually caused by either a mechanical overload or problems in the start circuit of the motor, which in turn is usually the contacts on the centrifugal switch. If your machine was easy to turn with the sanding disc this would rule out the mechanical overload.
IIWM I would start by removing the lower drive belt so the motor is not trying to spin any external load and see if the motor starts with no load. If the motor still hums but doesn't spin turn off power. At this point remove the motor, inspect the external wiring, identify the motor (probably Emerson 1-1/8 HP). If you have an air compressor you can try blowing sawdust out of the motor, the centrifugal start switch is near the opposite end from the shaft. This usually works, but you may need to more thoroughly clean the contacts or we may need additional troubleshooting steps. This would be a good place to report back to the forum with motor identification and results and we can assist further.
