Advice on how to fix window mechanism?

how can I fix mt upvc bathroom window lock?
the locking mechanism in my bathroom upvc window is broken: the square lock turns without affecting the up-and-down locking mechanism - How can I get to the mechanism? Do I have to remove the glass and if so how do you do that? For got to mention the window is in the lock position at the moment ie I can't open it...
Thanks for any advice..