Sounds like the torsion spring broke. You will need to call a tech to have it repaired. If you are strong enough to lift the door with the GDO disconnected - I wouldn't be! - you can lift it manually. The way to do that would be to get a helper and work it up first on a sturdy wood box, then another and finally up to a step ladder. Once you get it up most of the way, it should move relatively easily horizontally. Do all this very cautiously; it is dangerous. Once up high enough, you can block it open with a 2x4 or the like and drive out.