The garage door will not close
A door that opens but refuses to close is almost always the safety beams. The two infrared sensors mounted low on either side of the opening must see each other; a spider web, a bag leaning against one, a knocked bracket or a lens covered in dust breaks the beam and the opener refuses to close as designed. Most units flash an error code on the head unit while this is happening.
If the beams are clear and aligned, the next suspects are the down-travel limit setting and the close-force setting on the control board. A door that closes fully and then immediately reverses is being told it has hit an obstruction — usually because the limit is set slightly too low and the door is pressing into the floor.
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.