This is a particular problem that affected one of my kitchen windows and a bathroom window. The windows were installed cerca 1995 and the guarantee was rubbish – they just wanted to get out of fixing them by saying the only thing that is guaranteed is the glass. The bathroom one remains broken shut – cannot be opened as the common problem means that gravity sends the bottom lock to remain in place. This being the case turning the handle turns the centre mechanism but the ‘jaws’ you can see in the picture below never work to lift the metal lock up and out of its seat. This design flaw has meant that newer upvc windows designs do not use this kind of mechanism. The chances are that you are here to try and fix yours.
The first step is to get the window open – only then can you remove the screws from the inside face of the ‘sash’ (the framed pane). This is facilitated by removing the glass – so its a very hard step. The frame needs to be flexed out with tools hoping that it doesn’t get broken.
The second step can be to see if you can find the right size of mechanism replacement – then its a reasonably simple job to replace yourself this and bingo.
This is a picture of the mechanism that I needed to replace and chances are this is also what you need. If you need to get this part then contact me and I will get one sent across to you for a tenner.
Here are some links to help you get your window back functioning again.
A guy here tries to help you removing glass from the frames, remove glass from pain.
Remove the handle like this video shows.
Poor girl has the same issue here in this forum and it may be of help though some of the posters are confusing the issue.