In case your display arrangement or settings, e.g. when you have 3 screens and you are for some reason unable to reorganize them or change their settings, just reset your display settings:

For KDE Plasma:

rm -rf .local/share/kscreen

For GNOME:

rm -rf .config/monitors.xml

Then just log out and back in.

That’s it! Enjoy!

Sources:

https://askubuntu.com/questions/749333/how-can-i-reset-my-display-settings-through-terminal