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Power settings, turn screen back on

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?