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So most of what follows is about what not to do. Step back, keep every person and every vehicle out from under the door, and read the rest of this before you touch anything.
A garage door off track is a load problem, not an alignment problem
The track does not hold your door up. The springs do: a torsion spring on a shaft above the opening, or on a lot of older Valley homes, extension springs stretched along the horizontal tracks. Either way the load runs down a lift cable on each side to a bracket at the bottom corner of the door. The track only guides the rollers. So when a door leaves the rail, something in that load path gave out first. This looks like an alignment problem and is not one.
Do not run the opener, not even to test it
Your opener has a force limit that can stop it when the door binds. Do not count on it. It senses how hard the motor is pulling, so it reacts only after the door has been dragged out of square, and a door with a cable off its drum can run with less resistance than normal, so nothing trips at all. What gives is the door: the top section buckles, a hinge tears out of a panel, the track pulls off its brackets, or the opener strips its drive gear. Unplug it at the ceiling outlet if you can reach the plug safely, because the real risk is someone else hitting a remote.
Do not try to force the door back into the track
Levering a door back into the rail feels like the obvious fix. It is the single worst thing you can do. The door is still connected to loaded springs and cables, so prying on it can release that load all at once and bring the door down. A door section weighs enough to break bones, and the pinch point between a roller and the rail will take a finger off. Wait for someone who can take the tension out of the system first.
How to tell if a cable has come off the drum
Look from clear of the door: the doorway to the house, or outside the opening and off to one side. Never under the door, never in its path. From there, look at the top corners of the opening, at each end of the shaft the springs sit on. Each end carries a grooved drum with the lift cable wound into it. A jumped cable hangs slack, coils loose, or laps over itself instead of sitting in the grooves, and at floor level the door sits lower on that side. Do not touch the cable, the drum or the shaft, which stay loaded even with the door closed. If you cannot see it from a safe spot, leave it.
What puts a garage door off track in the first place
Four causes cover nearly all of it: a vehicle backing into the door, a broken cable or spring that let one side drop, rollers worn until they jumped the rail, or something left in the door path that stopped one side while the opener kept pulling the other. Around the Valley, dry rollers are a bigger factor than people expect. Fine grit packs into the bearings until the roller stops rolling and starts sliding. A sliding roller climbs out of the track.
How to secure the opening while you wait
If the door is fully down, lock the interior door between the garage and the house and leave the garage door alone. If it is stuck partway open, treat the garage as an open room. Move anything valuable out of reach, and park clear of the opening rather than under it. Do not pull down on the door to close the gap: blowing dust and summer storm wind are a nuisance, not a reason to get under a loaded door. Tell everyone the door is out of service and the remotes are off limits, and keep pets out of the garage.
When to call someone, and the part that really is DIY
Plenty of garage door jobs are genuinely DIY: wiping dust off the photo eye lenses, clearing the door path, changing a remote battery, lubricating rollers and hinges. A door off the track is not one of them, and neither is anything touching the springs, the cables, the drums or the bottom brackets. Those parts hold several hundred pounds of stored energy with the door closed, and there is one safe order: secure the door, release the tension, fix the cause, then realign. Call (855) 866-4555 at any hour, because a door stuck open is a security problem as well as a repair.