This is the issue with UPVC windows in that when closed all the mechanism
is out of sight so when things go wrong the default situation is the window
remains closed and the mechanism hard to access.
We had a window mechanism where the gearbox detached from the front plate
similar to the first video being only held in place by 4 small tenons that
were piened over. Fortunately it was a top hung window so it did not fall
into the extrusion after I removed the handle. What I found is if I left
the handle in place but not screwed to the mechanism it allowed enough
movement to get the gears engaged and the window to be opened.
If you suspect the same has happened to your mechanism don’t be tempted to
remove the handle immediately but loosen off the holding screws and tilt
the handle to try and engage the gears to open.
Richard