Hmm, weird bug in #KDE suddenly left me without main screen on my laptop with #KDEneon. Attaching an external screen made it possible to work around. Apparently a wider KDE issue, not only KDE neon, since this post on reddit describes the same for Fedora: https://www.reddit.com/r/kde/comments/1d9it4z/black_screen_after_kde_plasma_login_solution/ Although, I was not mirroring my screen. Deleting the json and then `killall ...` command described worked for me, but only after then restarting my laptop WITHOUT the external screen attached.