Liftmaster garage door openers work by remotes and a wall mounted panel. Like any other manufacturer's garage door opener, Liftmasters come equipped with motion sensors that are placed near the floor of the garage on either side of the garage door. If your Liftmaster garage door opener has stopped functioning as normal, there are a few different things to check to get it working again.
Pull down on the red cord hanging from the rails near the Liftmaster garage door opener to release the door, then open your garage door manually.
Attempt to close your garage door by pointing your remote at your Liftmaster opener. If it does not close, push the close button on the wall mounted panel. This will tell you if the problem is with the remote, such as a dead battery or the remote needs reprogramming. If the door still does not work, go to the next Step.
Move anything that may be blocking the sensors that are mounted near the garage floor on either side of the door. If nothing is blocking them, clean them with a damp cloth. Try to close the door using the remote and the wall mounted panel. If the door still does not operate, go to the next Step.
Check the breaker powering the Liftmaster garage door opener to ensure it is not tripped to the "Off" or "Neutral" position. Should the breaker be tripped to either position, turn it on and attempt to open and close the door using the opener.
Stand on a short ladder or stool placed under your Liftmaster door opener and make sure that it is plugged in. In addition, check the wires for any breaks or burn marks. If you see any breakage in the wires, replace them with comparable sized wire by cutting the bad section out with wire cutters and joining the two ends with a new wire and wire nuts.