Agree that it sounds like the spring broke especially since you say it will not retract at all.
It is a PITA to do. You have to basically remove the roller cover, pin the roller, remove the roller, pull the roller off the fabric, reinstall the roller, add turns to the spring (if it is not broken.... will take two people) and re-pin the roller, remove the roller again, reinstall the fabric on the roller, reinstall the roller, take the pin out, reinstall the cover.
I think I'm out of breath...
There may be other tricks but that is how the process is described in the SOKIII manual.