Power settings, turn screen back on

[Ulrich Marckmann]

On an older (windows XP era) laptop running Stretch with XFCE desktop I
have set the power settings to turn the screen off after some time, but I
cannot get it back on.
The touchpad, space bar, enter key does only turn the screen on in a sense
that it's on but black. I can increase/decrease the blackness with the
function keys, but I don't get the desktop.
Any idea on how to get the desktop back?
Thanks,
Ulrich