If the door is only slightly off and still closes smoothly, you can sometimes get a popped roller back into the track by hand - door down, opener disconnected, no tension on anything. That's about where the DIY line ends.
If there's a cable off its drum, a bent track, or any sign the spring has shifted, step back. A torsion or extension spring under load can take a finger or worse, and a bent track that gets forced back into rough alignment often fails again within weeks because the metal's already stressed. We'd rather tell you that up front than have you call us after a second failure that costs more to fix than the first one would have.