What model # is the Garage Door Opener? Did you remove the s/m cover (4 screws, 2 ea side) if I am correct with this being a Garage Opener? Did you see why the chain came off? Such as: I had a 1/3 HP Sears to recently come off due to a worn cable and about to break. This was a cable/chain combo vs all chain and the plastic pulley on the motor�s opposite end had worn badly (probably due to the chain getting too close to the pulley). I repaired with all chain and a steel sprocket on both ends (a better system IMO). The cable probably was off the pulley at one point, due to the slack I saw that went away while inspecting. loger

You need to remove the cover from the motor unit and find where the screw went. Then you will find the bearing for the gear & sprocket assembly is worn and letting the shaft lean forward. This is what causes the chain to come off. The part number for the gear & sprocket assembly is 41C4220A available from Sears parts and comes with instructions.