Eight times out of ten this is due to the lock failing.
If the lock has failed then normally to open without damaging the frame you need to call an expert.
I would advise someone who is an expert in the repair of PVCu windows.
We price all these works without calling out over the phone.
Anyone who knows his stuff should also be able to do this.
However it could just be the handle.
You could just hit the handle up with a hammer which breaks the key locking part.
If it is only the handle which was faulty is is now simply a case of unscrewing and refitting a new handle.
Thanks Cavan
Cavan Welsh Window Systems, August 2009