Rear seat belts have a locking feature that enables a child seat to be securely latched into place. If the seat belt is pulled out of its reel completely, the lock mechanism comes into effect. By then slowly releasing the belt back into the inertia reel, a soft "clicking" will be heard. If the belt is then latched in this position, you get the net effect of a "locked" seatbelt. I suspect this is what has happened to the OP's car.
The seatbelt should auto-release when it is completely retracted. If it's not, try hard as it seems, to coax a few millimeters back into the reel. This should free up the mechanism. Failing that, I would remove the belt from its anchor bolt beneath the carpet just ahead of the rear wheel arch.
Front seatbelts will lock in the fully retracted position following a crash where the pre-tensioners have been activated and must be replaced along with the seat buckle.
Hope this helps.
blue skies
tony