So you managed to unwind your spring.... There are two ways to fix this.  First way is to buy a new assembly, about $40 - $50 at a dealer, not what you want to hear right now but hey, at least the new one will retract like a... new one!.  The other way is a little tedious but will work.  After it unwinds you can lift the blue cover all the way off and get a good look at what's going on inside.  The spring and cover should stay together.  In the center of the coil at the inside end of the metal strip there is a part of it that is straight and not curved, this will go into the slot on the seatbelt roller.  When this happened to me I was able to get the spring into the slot and then I simply wound it up until it was pre-loaded like before then added a few turns and put the screws back in.  But it was much harder than what it sounds.  Because there was no tension on the spring, the first few turns the coil kept coming out of the slot.  After many tries I finally got it wound back.