Most likely it's the photo cells, they are little infrared boxes at the bottom of your garage door. One should have a red light on it, if it's flashing then the operator thinks there's an obstruction. In this case you should clean both boxes and make sure there isn't anything in the way. Things like snow, water drops or even cob webs can set them off. Once cleaned if it's still flashing try moving the one with the red light slightly till it goes on solid. Once the red light is on solid your door should close as normal.
If the red light is out then the box isn't getting power or it is dead. Trace the wire going from the box to the operator and look for breaks in the wire. If the wiring looks good most likely you need a new set. Call us for parts or service.
If the red light is on and not flashing, you can try unplugging your operator from the outlet waiting a few minutes and then plugging it back in to reset it. If your door still reverses you should call us for service.