Broken garage door springs
Springs are the single most common failure point on any garage door because they do the actual lifting. A torsion spring is rated in cycles — typically 10,000 for a standard spring, which is roughly seven years at four cycles a day, and considerably less on a household that runs the door eight or ten times daily.
When one breaks, the door becomes dangerously heavy and unbalanced. Springs are replaced in matched pairs for a reason: both have the same cycle history, and fitting one new spring against one worn spring leaves the door pulling to one side and the survivor failing within months. Full detail is on the garage door spring repair page.
Broken or slipped cables
The lift cables run from the bottom bracket of the door up to a grooved drum on the spring shaft. When a cable frays, jumps its groove or snaps, that corner of the door drops and the door sits visibly skew in the opening.
A skew door binds in the tracks, and forcing it under motor power bends track sections and can pull the door off its rollers entirely. Cables are always replaced as a pair and the drums reset so both sides wind evenly — see garage door cable repair.
Damaged or misaligned tracks
Tracks take the punishment when a vehicle nudges the door, when a roller seizes and gouges the channel, or when a bracket works loose over years of vibration. Once a track is out of true, the door grinds at one point in its travel, sticks halfway, or jumps a roller out of the channel altogether.
Minor distortion can be straightened in place. Anything creased, split at a joint or bent at the radius needs the section replaced and the whole run re-shimmed — the detail is on the garage door track repair page.