Do a search theres lots of threads covering this very common problem It's usually a spring that breaks I believe if you prise the green oval out of the rear handle you can get at the lock mechanism to open the door. But like I said earlier a search should turn up the solution to your problem.
I had the same thing happen. LR will try and sell you the whole lock replacement for loadsofmoney! But go on ebay and buy a spring. Chances are it is a small spring inside the lock. Wrorked for me and at less than a fiver. Good luck.
mine got stuck a day or so ago ..........
make sure the opening handle returns to its proper position this was all the problem was with mine !!!!!!!!!!!!!