There are several reasons why your UPVC window won’t open. If the handle is turning but nothing happens it is likely that the mechanism inside the window is broken. If the window is jammed up we have several techniques to get it open and repair/replacement of broken parts.
It is common to be called to repair a window to find oil/WD40 squirted everywhere, the customer having tried to cure the problem. No amount of WD40 is going to make something like the examples we’ve removed below work again.
Typically window espags and gearboxes are made of brittle die cast metal. They can and do break with little provocation. They can normally be replaced fairly cheaply (compared to replacing the entire window).