Wont coil back into its housing, so trimmer wont start. Has compression though

Hi, this a cut&paste from elswhere but it applies to your problem:
how to repair a recoil spring: whats missing is this, you should always wgear eye protection when dealing with springs of any kind.
remember safty first. :)

Remove the recoil shroud and take a look at the assembly.
You might have a broken spring or just a rope that has slipped out of the reel.
The spring usually breaks at one of the end tabs. If so, you will need to get a replacement spring and carefully wind that in, before re-assembling the rope reel. You might wish to replace the rope also, if it shows signs
of wear.

You will need to remove the entire cooling shroud. There should be 4 bolts on the sides. You might need to remove the fuel tank also. Pre-wind the recoil reel by hand [about 6 turns, to set the spring tension] and clamp it with Vise-Grip pliers. Remove the old rope. Thread
the new rope through the shroud housing eyelet and to the reel hub. Let the reel re-wind itself until it is full. If there is not enough spring tension you can wind it a couple of turns more by slipping the rope around
between the reel and shroud. Re-clamp the reel. Cut the rope to length and thread it though the handle. At all knots, burn and melt the rope with a
lighter to prevent untying. Re-assemble.
(special thanks to "Micheal"the author)