Off The Track
A door jammed at an angle with rollers out of the track is an unstable load, and forcing it back is how people get hurt and panels get destroyed.
Short answer
The usual causes are a broken cable, hitting the door with a vehicle, a bent track, a failed roller, or an obstruction the door closed onto. Do not operate the opener and do not try to lever it back. The door needs supporting and the cause fixed, not the symptom corrected.
Why it comes off
A broken or slipped cable, which lets one side drop and pulls the rollers out of the track.
A vehicle hitting the door, which bends the track or the bottom section.
A bent or loose track, frequently from a previous impact or from loose mounting fasteners.
A failed roller or a broken hinge, which lets a section wander out of the track.
Closing onto an obstruction — a bin, a bike, a snow shovel left in the opening — which lets one side hang up while the other continues.
And the opener force set too high, which pushes through a problem that should have stopped the door.
Do not force it
A door off its track at an angle is held by cables and rollers that are no longer doing what they were designed to do, with the spring tension still in the system. Levering it back puts more load into a compromised assembly, and it either destroys the panels or drops the door. Secure it and leave it.
What to do
Stop using the opener immediately, and unplug it so nobody else tries.
Do not try to lift or lower the door by hand.
Secure it where it is if you safely can — clamps or vice grips on the track below a roller on each side.
Move vehicles out of the way of the door path if that can be done safely, and otherwise leave them.
Keep the kids and any animals well clear of the space.
And call somebody, because this is a same-day repair rather than a wait.
The repair
Support the door and release the tension safely before anything is moved.
Find and fix the cause: cable, roller, hinge, track, or an opener force setting.
Straighten or replace the affected track section, because a bent track will put the door off again.
Replace damaged rollers and hinges, and check the rest while access is there.
Check and replace a damaged bottom section where a vehicle impact has crushed it.
And rebalance the door and reset the opener force and travel limits, which is the part that stops it happening again.
Common questions
Can I put the door back on the track myself?
No. The spring tension is still in the system and the assembly is compromised.
What usually causes it?
A broken cable, a vehicle impact, a bent track, or closing onto an obstruction.
Is it urgent?
Yes. It is an unstable load. Secure it and get it dealt with the same day.