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A broken torsion spring, almost every time. The springs carry the door's whole weight, they are under enormous tension, and they are the one part not to attempt yourself.

Usually a stripped gear or a sheared trolley. Often repairable, and worth pricing against the age of the opener.

Safety sensors misaligned or dirty, or the force setting drifted. Sometimes a two-minute fix; sometimes the sensors are telling you the door is binding.

Worn rollers, loose hardware, or dry bearings. Cheap to service now, expensive once it damages the track or the panels.

Usually a broken cable, a bent track, or something hit it. Do not run the opener again -- that turns a repair into a replacement.

Single panels can often be replaced if the model is still made. Otherwise it becomes a question of whether to match or replace the door.

Perished bottom seal, or a slab that has settled. Straightforward to fix and it stops the garage acting like an open door.

Old doors are heavier, noisier, uninsulated and lack modern safety reversing. Worth knowing what a replacement costs before the springs decide for you.