sounds like the mechinism or the handle pin has snapped
unscrew the handle and remove it if the pin has snapped remove it and insert a flat head screwdriver and unlock the window.
If the mechinism has snapped in the locked position you will need to de-glaze the window to access it.
You will find all the parts from any double glazing firm, have a look in their skip,there should be all the parts you need.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!