When a UPVC window handle won't open, it's usually because the plastic tap, which should be released once you push the button in, has failed. Opening a window handle which has failed is usually straightforward, either using a credit card or thin piece of plastic by forcing it into the gap and pushing the tab down. Instead of repeating the same process, you can tape the tab down, using electrical tape or something similar.
If you need to replace the handle, watch this video: • How to Replace a Broke...