A torsion spring under tension that's lost its temper from age and cold doesn't ease off - it lets go all at once, and on a double door that's two springs working together, so one failing puts the full load on the other. That's the most common emergency call, and it's also the most dangerous one to touch yourself.
Cable fraying is the slower version of the same problem: you'll usually get a warning in the form of a door that drifts or binds before a cable actually snaps. Rollers going bad on a cold morning will bind in the track and make the opener strain, which over time burns out the drive gear.
Here's the honest limit: if the door itself has warped, or the track has rusted through at the bottom where snowmelt sits against it, replacing a spring or a cable doesn't fix the underlying problem - at that point you're better off replacing the door than paying to patch a frame that's failing. We'll tell you when that's where you are instead of selling a repair that won't hold.