When trying to get nvidia display drivers installed and configured, the screen would sometimes lock, or go black, or become non-functional.
How to restart X session (KDE) under systemd helped with the background material.
Method One, via the gui: Applications -> Settings -> System Settings -> Input Devices -> Keyboard -> Advanced -> Key Sequence To Kill X Server: ctrl + alt + backspace
Method Two, via the command line:
~$ sudo systemctl list-units | grep -i manager ModemManager.service loaded active running Modem Manager NetworkManager.service loaded active running Network Manager sddm.service loaded active running Simple Desktop Display Manager systemd-udevd.service loaded active running udev Kernel Device Manager udisks2.service loaded active running Disk Manager ~$ sudo systemctl restart sddm.service