We recently got called out by a customer who could not lock their window. It was a tilt and turn window and the handle would not move to lock the window after closing it. When we arrived the customer showed me what they thought was part of the hinge which had fallen off. It turned out not to be part of the hinge but was actually a small part which attaches to the frame of the window. When the window is closed part of the locking mechanism pushes against this part on the frame, if it isn't pushing against it the handle will not move. The customer had looked all over the window to try and see where the part belonged but couldn't see any screw holes where it had been fitted. This was because it had never actually been screwed in place and had just fallen out when it became loose. The fitter who fitted the window must have just forgot the screws to hold it in place. It might be worth checking your doors and windows for loose (or missing) screws.