There's a real limit here, and it's worth saying plainly: a torsion spring under tension can put someone in the hospital if it lets go while you're working on it. We don't walk homeowners through spring replacement over the phone, and we won't pretend it's a weekend job. That's the honest limitation on this page - some of this door is not a DIY repair, full stop.
What you can safely check yourself: whether the opener is plugged in and the breaker hasn't tripped, whether the manual release cord has been pulled (common after a power outage, and it disconnects the opener from the door entirely), and whether anything - a bike, a bin, a stray toy - is breaking the photo-eye sensor beam near the floor on either side of the track.
If the door is off its track, the spring is visibly broken, or a cable has snapped, stop. Don't cut the tension, don't force the door up by hand, and don't let anyone stand under it. That's exactly the call we drive out for.