Try this: Plasma session goes blackout after leaving it AFK (workaround exists) - #19 by vini.nu
Try changing TTY back and forth, slowly, sometimes helps:
Ctrl+Alt+F1
Ctrl+Alt+F2
Ctrl+Alt+F3
Ctrl+Alt+F4
Try this (for hibernation): Plasma session goes blackout after leaving it AFK (workaround exists) - #27 by vini.nu