If the door is only partway off - one roller out, the panel still square - you can sometimes walk it back into the track by hand with the opener disconnected. That's a two-person job at minimum, and it only works if nothing's bent. The second a track bracket has pulled away from the framing or a section of rail is visibly curved, hand-fixing it just forces the same roller back out again within a few openings.
The honest limit here: if the door came off track because a cable jumped the drum or a spring already let go, do not touch it. A door under spring tension that's also off its track can come down fast, and there's no safe way to wrestle that back into place without the right bars and clamps. That's not a DIY save - that's a call.