1. Check to see if the pulley is turning.
2. If not, check to see if the motor shaft itself is turning.
Lots of non-motor possibilities: the pulley may be stripped, the belt may be stretched, or the motor may have shifted, releasing tension on the belt, or (less likely) may have shifted in such a way as to bind.
If the problem is with the motor, it’s a little more difficult to diagnose: could be a problem with bearings, the shaft could be seized, or about a dozen other things. If the motor is on but the shaft won’t spin, UNPLUG IT, then try manually turning the shaft to see if it is seized. If it won’t turn, it likely has bad bearings, dirt in the bearing/slipring, or is otherwise damaged. If it does spin by hand, you’re likely just binding due to the motor rotating on its plate. (Less likely, but possible.)
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