Check to make sure the trolley isn't disconnected from the door to the track. If someone pulled the release cord this would happen. If that's not it, the cause will either be a broken chain (which I would think you would see) or the drive gears are broken/worn.
If you want to visually inspect he gears make sure to unplug the opener before removing the cover. Once you have the cover removed to where you can see the gears, if you cannot see any damage to them plug the unit back in and try to run it. When it starts to turn the motor pay attention to the gears to see if the worm gear is moving the drive gear. If it is, notice the sprocket to see if it's turning or not.
I'm pretty sure the gear set will be the problem.
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