Most likely there is a safety overload switch that will stop if the rollers on the track ( the steel that the door rolls in) is dry and needs to be greased so it can roll in the tracks freely also check the chain for the same, the chain drive should be free of any junk stuck on it ,the chain should not be to tight or twisted in any way, if it seems to be leaning one way or the other twist it straight with a pair of pliers also be sure its power source is not share with any other appliance. let me know of your progress.