So yesterday I tried connecting an external monitor to my nVidia proprietary driver KDE laptop. It went meh, as the external display is so much lower resolution (and so much larger) than the configurations on the laptop make the monitor look like a VGA blown up to ridiculous proportions. Oh well, unplugged the monitor this morning, closed the laptop and headed to the cafe to work.

Oops, black screen on reboot, could switch to virtual consoles, but no way I could find to get KDE back up. Rebooting didn't work, would come back to exactly the same broken config. xrandr didn't seem to be able to reset anything on the display. So, the solution:

rm ~/.local/share/kscreen/*
sudo reboot

And we're back in business... minus an hour of work-time.