Hi!
I apparently fixed the problem. I did following:
Inspired by this : https://www.dropbox.com/s/dc6nmx0pa2g5fgz/Blackscreen.mp4?dl=0 (from this threat Screen can not be activated by pressing the power button)
i pushed strongly but smoothly this part of the phone*, and since then no more problems. But now, i downloaded two apps, one to unblock the screen moving the phone and one which changed my power button (just in case if this happens again).
*But not where the screen is, instead i pushed the back of the phone. A bit under the cam, where the `cable´goes from the button to the other end.
Best,
Pascal.