If The door starts to close, then reverses and the opener light blinks.
Then Photo-eye alignment.
The sensors near the floor are out of line or dirty; a clean and re-aim usually restores normal closing.
Springs
Adjusting spring tension is fine work. A quarter turn changes how the door rests at halfway, and the aim is a door that stays where it is left. We adjust, test by hand, and adjust again until it does.
Garage Door Roller ReplacementThe door shudders as it rises, worst where the track bends from vertical to horizontal.
Garage Door Hinge ReplacementThe roller at the top of the second panel is leaning outward and only half of the wheel is still in the track.
Garage Door Track Repair or ReplacementBacking out this morning, the mirror caught the track beside the door.
Garage Door Cable ReplacementSomeone left the snow shovel leaning in the opening and the door came down on it.
Torsion Spring ReplacementLooking up at the coil above the door, you can see a clean split with a gap of several centimetres between the two halves.
Extension Spring ReplacementOpen the door by hand and it drifts back down on its own, a little at first and then all at once.
Garage Door Balance AdjustmentYou pulled the red cord to move the door by hand during the power cut, lifted it to about waist height and let go.
Which fix fits?
Tap a situation. The answer opens here.
If The door touches the floor and pops back open.
Then Close-limit adjustment.
The opener thinks it hit something; setting the travel limit correctly stops the bounce.
If The door closes but leaves a gap along the bottom.
Then Bottom seal and limit setting.
A compressed seal or a short travel limit leaves daylight; both are simple to correct. Garage Door Bottom Seal Replacement