How to fix a garage door that is lopsided from something being stuck under it?

First, take a look at the door as it is now. Is it still rectangular, or is it tzoidal from the stress of being warped? If it's tzoidal, then you must reshape it -- you'll probably have to add a brace of some sort to hold it's corrected shape. If it has those long springs which swing from the back wall of the garage, you might help the situation by opening the door, then tightening the spring on the "low" side of the door. DON'T EVER disconnect or tamper with the springs when they're under tension -- mayhem will certainly ensue. If your door is wide (sixteen or eighteen feet wide), and if you have two springs wrapped around the pipe looking rod on the wall above the door, you can adjust the spring on the low side -- you'll have to make a tool to stick in the hole to turn it, then remove and reinstall the little pin which keeps it tensioned. This won't help if your garage door has only one such spring. This type is very hard to reach with the door opened. Don't mess with garage door springs if you don't know what you're doing. Things can get out of hand in a split second, and people do get hurt -- sometimes badly. Please be careful, and good luck with your project.