For several reasons, you may find yourself opening the garage door by yourself. Like opener parts failure or loss of power supply, then you pull the emergency cord to release the door opener. But later, you need to re-engage it. You can resent the trolley in two systems: manually or automatically.  Put in the cord spring lever in the connected position and pull up and down the door until the track is re-connected. For the automatic system, you can use the garage remote that reconnects with the trolley automatically. So following these simple teachings, you can reset the trolley.