Motor hums but nothing happens

Pull down on the manual release rope. It may be hard to pull.

Use the emergency release handle to release the door. Run the opener to the open position until it stops. Close the door until it stops. Now re engage the the door.

Release the door from the opener by pulling down on the rope. It will be very hard to pull. This should release the tension on the chain but I think you need professional help with this opener. It sounds like it is badly out of adjustment. The chain coming off is an indication of a worn bearing that will have to be replaced.

My plastic gear stripped soon after a spring broke. I think the broken spring put a huge load on the opener that day and ruined the gear. Replacement wasn't too hard. It will be obvious when you remove the cover if the gear is stripped. Here are instructions:
http://tinyurl.com/gearfix

The humming is the motor trying to turn. Remove the drive end cover from the bar and chain--back off the chain adjustment CCW several turns. If the saw has a chain brake, release it. When the cover is loose, remove the bar and chain, inspect the entire mechanism for any damage--particularly the chain itself. Run the motor without the bar and chain, and check for oil oozing out of the side port in the motor case where the bar fits. To reassemble, fit the bar in place after cleaning the small oil passages in the drive end, then fit chain over the sprocket. Pull the bar outwards to tighten the chain, then fit the cover and fasteners finger tight. Adjust the lower run of chain up to the bar (the lower run of teeth cut toward you), but not so tight that you can turn the chain by hand. Tighten the fasteners and recheck chain tension. Chain saws don't seem to like cutting hedges and other small branch, but tough bushes and the like. Make sure the chain teeth are properly sharpened. Hope this helps!

Check to ensure one or more of your garage door springs isn't broken (they last about 10 years). Most doors, especially double wides, have two or more springs. If one breaks, as they ultimately will, the motor won't have enough mechanical assistance from the spring to lift the door; however, gravity and one good spring will certainly close it.

If the mower drive belt is extremely loose the spring that holds it tight may have broken,check to see if it is broken before you replace the belt

If your door is stuck in the open position you need to see if the trolley is jammed against the bolt just in front of the motor unit. If it is, loosen the chain enough to remove the tension from the trolley. Run the motor until the door closes and then locate the 'up limit' on the side of the motor unit. Turn this control 2 full turns in the opposite direction of the arrow printed on the cover. Tighten the chain.

You should check the gears inside the box, After years , the gears get dry and die, you can by online or from Sears the kit and replace the old gears. I just changed mine...not too difficult :) .

I found the chain extremely tight on the sproket. The mechanism had stalled hard against the stop bolt. I loosened the chain tensioner and the motor operated properly. Reset the limit adjustment so the mechanism stops a couple of inches from the stop bolt. Door is working fine now.