A drained battery is the most common reason why a remote control stops working.  Replace the battery in the remote control with a fresh, newly purchased battery.  Lift-Master remotes take a 3 volt battery, “CR-2032” – these are normally available at any drugstore or hardware store; we also stock these batteries at our shop.  If the remote 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.