Take the cover off and I'm sure you will find the drive gear is stripped. It isn't all that hard to replace and the new gear comes with instructions. The part number is 41A2817.

Does this model use sensor/eyes (the kind that are mounted near the floor)?
How does it know to reverse itself if something obstructs closing it?

This model does not have door sensors. When the door meets an obstruction it stalls the motor, the logic board senses this and reverses the door to the open position. Because it doesn't have door sensors the manufacturer no longer supports it.