I am guessing here that the spring unwound. Is that the case?
Or are we just talking about the doohickey that attaches the door to the door opener. (I.e., the attachment to the belt/chain/whatever).
If it's the latter, it's probably simple and you have some sort of mechanism that latches onto the chain/belt. Mine is twitchy and has to be "just so" to grab on. There is usually a rope/chain that releases/attaches it.
If it's the former, there are going to be a whole bunch of people that say "OH MY GOD! DON'T TOUCH IT." If that scares you, call someone.
I've unwound/rewound my spring several times and I'm still breathing. You'll need a couple of 2 ft sections of steel -- probably about a half inch thick. You'll need to figure out how many turns your spring needs (based on the spring size and the weight of the door). It likely has a little 4-socket turner with clamps that clamp onto a center bar. Effectively you'll need to use the 2 steel bars as levers to hold/move the turner. Hold it with the steel, loosen the bolts so it will turn and give it a quarter turn, put in 2nd steel bar, repeat.... until you've got enough turns in the spring.
IT IS A LITTLE DANGEROUS. If you let go, that spring will send those steel bars flying across the room (and through anything in its path -- possibly even you.) Once you get it where you want it, tighten down the 2 bolts and you're there.
This is probably a pretty abbreviated description. Look for a video on youtube ... once you see it done, it makes sense.