One of the window handles (espagnolette) on a conservatory fanlight would not turn. It wasn't locked, but pushing on the central lock button, which usually releases the catch, didn't work. It would turn maybe a millimetre, but that was it. I tried it with the key in to see if that helped, but no joy.
I had a couple of spare handles, so examined them to see if it was possible to access the catch. I tried pushing in various thin pieces of metal and needles to see if I could get to the plastic piece which wasn't releasing, but that also did not prove possible.The design of these handles doesn't help; to aid security, only one screw is visible and removable when the handle is in the closed position. The other is hidden under the lock, so it is not possible to remove the lock from the window unless it's in the open position - Catch-22!
In the end I was able to get a junior hacksaw parallel to the handle to cut off the small "lip" which stops the handle moving too far when closed. That just allowed me to get to the plastic piece, push it, and turn the handle. This is a close up of the handle after I removed it from the window showing where the plastic had snapped: