Detachable towball stuck in car

I had a detachable towbar fitted to my car a couple of years ago. I haven't been using the caravan much at all this year, but recently took it out for a run. 

The towball was difficult to insert, but I managed to get it in. On returning I found that the release mechanism had stuck and won't turn at all. This means I can't remove the towball from the towbar.

Can I do anything to free it up, or can it even be fixed?