You will probably find the recoil spring is jammed. My SGT24 did exactly the same, I put it down to poor quality equipment.
To examine it, remove the four screws to remove the plastic cover. Watch out! because the spring may jump out. You will find the spring has jammed against the roller mechanism. Slackening the screws enabled the cord to retract, but the spring did jam.
I am looking for a replacement spring assembly, if I source one I will let you know
peter hall
July 2007