Sounds as though the gear that dives the chain has come loose from the drive
shaft of the motor.

It probably has a set screw that has loosened.
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I have a 4-5 year old Sears 1/2 hp garage door opener. It had been
working fine, but a few days ago it malfunctioned. When I press the
control to open it, I can hear the motor running, but the gear that
drives the chain doesn't turn. Any suggestions on how to fix it?