UPVC windows may become difficult or impossible to open / lock. Reasons for this may include poor installation, poor alignment, dropped window, lack of lubrication, wear and tear etc.
Window and kitchen windows tend to break or jam more than other windows. This is due to the increased moisture in the air in these rooms.
The gearbox above is one of the most common makes that we replace. They break because of poor fitting / maintenance. The window had been jammed closed for months. You can see how badly corroded and caked in crud this has become over the years. One of the mounting screws had to be drilled out because it was so seized.
An occasional application of lubricant to the moving parts of your windows can prevent this happening.