If the door is off track but the panels aren't bent and the cables are still under tension, some of this is within reach for a careful homeowner - loosening the bracket, easing the roller back onto the rail, and retightening. But if a cable has come off a drum, if a spring is holding the weight of the whole door, or if you can't tell which one it is, stop. That's not a place to guess. A door under spring tension that lets go while you're working under it can put someone in the hospital. Given how much of the hardware in this area is original to a house built decades back, brackets can also just fail - pull clean out of a stud - the moment you put load back on them, so a fix that looks finished can come apart again within days if the mounting underneath isn't actually solid.