I think there must be a fault with the lock if the arm is not turning, or the arm has rounded off on the lock shaft. I doubt the nut has come off, as you would be able to lift it off.
You have two choices.
1. Drill the centre of the lock out and buy a new lock.
2 Break into the top segment- undo the nut and remove the arm.
Sounds like the slip lock has loosened or else the lock itself is buggered. Either way Paul is right . Just drill the lock. Better than breaking the top segment or you could be replacing both parts.