Stuck, Now
A door stuck open is a security problem and a door stuck shut is a vehicle problem, and the first thing in either case is to stop operating it.
Short answer
Stop using the opener. If it is stuck open, secure the opening — clamps on the track to hold the door, or secure the service door into the house and move anything valuable. If it is stuck shut and the vehicle is inside, the release cord disconnects the opener, but only lift it by hand if the springs are intact.
Stuck open
Stop using the opener, and unplug it so nobody else tries.
Secure the door where it is with clamps or vice grips on the track below a roller on each side, so it cannot come down.
Lock the service door from the garage into the house, which is the actual security exposure.
Move anything valuable out of the garage or out of view.
Park a vehicle across the opening if that is practical.
And treat it as a same-day call, because an open garage overnight in a London winter is also a frozen-pipe risk.
The release cord
The red cord disconnects the door from the opener so it can be moved by hand. That is safe where the springs are intact and the door is balanced. It is not safe where a spring has broken, because then the full door weight — well over a hundred kilograms on a double door — is on whoever is holding it. If the door felt heavy or there was a bang, leave it.
Stuck shut with the vehicle inside
First establish whether a spring has broken: was there a bang, and does the door feel heavy if you take any weight on it at all.
If the springs are intact, pull the release cord and lift the door by hand — it should feel light and should stay where you put it.
If the springs are broken, do not lift it. The door needs supporting properly and that is not a two-person lift.
If the opener is the problem and the door is sound, the release cord is the whole answer and the vehicle comes out.
Secure the door down and reconnect the opener only once the fault is fixed.
And if the door is partly open and jammed at an angle, leave it and secure it rather than forcing it either way.
The power cut case
A power cut leaves the opener dead and the release cord is the answer, which is why knowing where it is matters before you need it.
An opener with a battery backup keeps working, which in a London ice storm with the garage as the main entrance is the case for having one.
An exterior keyed release is available for a detached garage with no other door, and is worth fitting where that applies.
Reconnect the opener by moving the door to the point where the trolley re-engages, or by pulling the cord toward the door, depending on the unit.
Check the door and the opener after a power cut, because a door left part open and then powered up can bind.
And know where the release cord is — which sounds obvious and is the thing people discover they do not know in the dark.
Common questions
My door is stuck open. What first?
Unplug the opener, clamp the track so the door cannot drop, and lock the service door into the house.
Can I use the release cord?
Where the springs are intact, yes. If a spring has broken, no.
What about a power cut?
The release cord, or a battery backup opener — which is worth it if the garage is the main entrance.