i've had success with vicegrips clamped to the track,pulling down on the door giving some slack on the messed up cable drum. then winding the cable onto the drum manually then removing the vicegrips. good luck!!!

You use the vice grips to keep the door from falling down?

You need to hold the door up with some type of a prop, (2x4) (I've also used the vice-grip in the track trick, but this has been known to fail at an inopportune time)
turn the shaft on the spring/reel mechanism to take tension off of the cable,
inspect the cable for damage (a broken strand calls for replacement),
shift the cable back onto the reel, taking care not to nick or otherwise damage the cable,
ensure the entire cable is neatly wound around the reel without overlapping itself,
check the other reel to ensure it has not fouled while tension was removed,
reapply cable tension,
recheck the reels for cable fouling,
remove the door prop.