I think your problem lies with the friction hinge(basically the window hinge)There is a small black plastic section that slides in the stainless track which holds all the arms of the hinge together. It is a weak point and if it breaks, what has happened to you usually occurs. To close the window, you need to go outside and push the edge that wont locate back into the frame while someone inside faffs around with the handle (leading edge ) side. You will probably have to close it properly before it will open again. When you get it open, inspect the aforementioned sliders for breakage. The fix is cheap and simple, just replace the hinges with ones from screwfix or toolstation - less than a tenner a pair. Be aware that as well as different lengths of hinge, 250,300,450 etc there are different stack heights of 13 and 17mm. Older windows use a 13mm, newer 17mm - it is just the height, when closed together, of the hinge. Dont be phased by the job, its a simple fix with just few basic tools. Good Luck