A drained battery is the most common reason why a keypad stops working (keypad batteries need to be replaced fairly often as the batteries are exposed to temperature extremes).  Replace the battery in the keypad with a fresh, newly purchased battery.  Lift-Master keypads take a 9 volt battery, available at any drugstore or hardware store; we also stock 9V batteries at our shop.  If the keypad does not begin working immediately it may need to be re-programmed – feel free to give us a call if you need instructions on how to re-program.