A Broken Cable
A cable going means one side of the door loses its support, which is why the door ends up crooked and frequently off the track.
Short answer
The cables transfer the spring force to the bottom of the door. When one breaks, that side drops, the door goes crooked and usually comes off the track. The common causes are fraying at the bottom bracket, corrosion, a cable that came off the drum, and water and salt pooling at the bottom of the door — which in London is a winter issue.
Why they break
Fraying where the cable turns at the bottom bracket, which is the highest stress point and where most failures start.
Corrosion at the bottom of the cable, which sits in whatever water, slush and road salt collects at the door threshold.
A cable that has slipped or unwound from the drum, usually after the door was let down too fast or after a spring failure.
Wear from running against a worn drum or a misaligned pulley.
A spring failure, which puts the whole door weight on the cables suddenly.
And age — a cable is a wear item on the same clock as the springs.
Salt at the threshold
In a London winter the bottom of the door and the bottom cable attachment sit in melted snow carrying road salt from the vehicle. That is a corrosive environment, and it is the reason bottom-bracket cable corrosion is more common here than the mechanical wear would suggest. Rinsing the threshold and the bottom of the door in spring is a free maintenance item.
What to do when one goes
Stop using the opener and unplug it.
Do not try to operate the door by hand, because one side is unsupported.
Secure the door where it is with clamps on the track if you can do so safely.
Keep people and vehicles out of the door path.
Do not attempt to re-seat a cable on the drum, because that means handling the spring tension.
And expect the repair to include the other cable, because they are the same age.
The repair and what to check
Replace both cables rather than one, matched to the door height and the drum.
Check the drums, which can be damaged where a cable has come off and run against them.
Check the bottom brackets, which on an older door are a corrosion point — and which are under spring tension, so they are not a homeowner item either.
Check the springs, because a cable failure and a spring near the end of its life frequently go together.
Check the bottom section and the rollers for damage from the door going crooked.
And rebalance the door and reset the opener, which is the finish of the job.
Common questions
Why did the cable break?
Usually fraying or corrosion at the bottom bracket, which in winter sits in salty meltwater.
Should both cables be replaced?
Yes. They are the same age and the same cycle count.
Can I re-seat it on the drum myself?
No. That means handling the spring tension.