If you hear the motor humming and not starting at all it most likely has a severe friction problem with the drum or motor. It could possibly have some type of garment jamming the blower wheel preventing the motor from starting too.
Open the dryer door and manually rotate the dryer drum counter clockwise (CCW). It should take very little effort to rotate the drum. If it's hard to rotate that could explain why the motor hums instead of starting up. If it turns freely, shut the door and see if will start. If it still hums and not start it may have bad motor or a low voltage supply to the motor.
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