We have PVC windows that open inwards 2 ways -from the top and from the side ( tilt and turn ). Somehow we have managed to open the window from the side when it was already open tilting inwards from the top !!
And now it won't close as it seems jammed and the window won't tilt back so we can lock it in place.
Our windows do this if we're not carefull. You need to open the handle one way (eg so that the window tilts) then push the corner in for the other way (eg push it so that it is in the position for being turned open), finally turn the handle the other way (eg so that the window now turns) and then shut it.
If that doesn't work try switching tilt and turn in my examples. Hope this helps. If it doesn't I'm afriad I don't know how to fix it.