After upgrading Kubuntu from 16.04 to 18.04 the system seemed to work ok except for problems with multiple monitors.
But after a reboot X started crashing on start up (that's another problem). I removed the Nvidia card, and now I can get to the login screen. But after logging in the screen goes blank with just the mouse cursor visible.
I can Shift+Alt+F2 to switch to a tty. I've tried
xrandr from there but can't connect to the X display.