The Spring Went
A loud bang from the garage and a door that will not lift is a broken spring, and it is the one garage door repair that should not be attempted at home.
Short answer
The springs carry the entire weight of the door — a double door is well over a hundred kilograms. A broken spring means the opener and the cables are now carrying that load, which they are not designed for. Do not use the door, do not disconnect the opener and try to lift it, and do not attempt the spring yourself.
What has happened
The spring counterbalances the door weight, so the door feels light and the opener only has to overcome friction.
With a broken spring the full weight of the door is on the opener, the cables and the drums.
The opener will either fail to lift it, strain and stop, or in some cases lift it part way and then lose it.
The cables can come off the drums or snap, which is how a door comes down unexpectedly.
A door that comes down from height with nothing holding it is a serious injury risk, and it has happened to people and to vehicles.
And the bang people hear is the spring letting go, which on a torsion spring is violent.
Why this one is not a homeowner job
A torsion spring holds an enormous amount of stored energy and it is wound and unwound with bars under tension. Getting it wrong causes serious hand, face and head injuries, and it happens to people who thought they had read enough. This is the one garage door job that is genuinely not a homeowner task, and saying so is not a sales line.
What to do now
Stop using the door. Do not try the opener again, because that strains the opener and risks the cables.
If the door is up and you can get the vehicle out, leave it up and secure it with clamps or vice grips on the track below the bottom roller.
If it is down, leave it down. Use another door if there is one.
Keep children and pets out of the garage.
Do not disconnect the opener and try to lift it by hand, because without the spring that is the full weight.
And call somebody, because this is a same-day or next-day repair rather than something to live with.
The repair
Replace in pairs. If one torsion spring has broken, the other is the same age and the same number of cycles — replacing only the broken one means another visit soon.
Match the spring to the door weight and height properly, because the wrong spring means a door that is unbalanced and an opener that is overworked.
Expect the cables, the drums and the bearings to be checked at the same time, because a spring failure frequently damages them.
Expect a higher cycle-rated spring to be offered, which costs a little more and lasts considerably longer.
Expect the door to be balanced afterwards and the opener force settings checked.
And expect the whole job to be an hour or two rather than a day.
Common questions
Can I replace a garage door spring myself?
No. Torsion springs hold enormous stored energy and cause serious injuries.
Should I replace both springs?
Yes. The second is the same age and cycle count as the one that broke.
Can I use the door until it is fixed?
No. The opener and cables are carrying a load they are not designed for.