When a retractable reel has had its hose fully pulled out (past the end of hose marker), then it can lock. This will stop the reel from retracting. While this sounds very basic, there's only one thing to try - and that's some short sharp tugs.
Here's how to do it:
* Standing as close to the mouth of the reel as possible, take a stance as if you are going to have a tug-of-war with the reel.
* With 2 hands on the hose do do a few short sharp tugs (3 or so - similar to starting a old Victa lawn mower) at the point where the hose has stopped, to see if it will start to retract.
Please ensure that you do this close to the hose case, and not from out in the garden
If there's been any noise (bangs or cracks?) made by the reel either before, or during you trying to retract it may be an indication that something is broken in the reel - please use caution when attempting the short sharp tugs because if the retraction 'takes off' you may find the hose moving very quickly back into the reel.