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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.

Any suggestions?

The answer seemed to be to remove and purge the nvidia drivers.

However I now have the nvidia card working after following advice in
this post

Unfortunately I'm still limited the one graphics card with DVI and VGA, whereas in 16.04 I was successfully running 3 monitors via two cards.

I had similar issues where the upgrade from 16.04 to 18.04 resulted in blank screen.

The key to finding the problems was reviewing the errors in the file /var/log/syslog

gnome-session[2851]: X Error of failed request:  BadValue (integer parameter out of range for operation)

gnome-session[2851]:   Major opcode of failed request:  154 (GLX)

gnome-session[2851]:   Minor opcode of failed request:  3 (X_GLXCreateContext)

gnome-session[2851]:   Value in failed request:  0x0

gnome-session[2851]:   Serial number of failed request:  19

gnome-session[2851]:   Current serial number in output stream:  20

gnome-session[2851]: gnome-session-check-accelerated: GL Helper exited with code 256

gnome-session-c[2979]: Couldn't connect to accessibility bus: Failed to connect to socket /tmp/dbus-4UdC671MtT: Connection refused

gnome-session-c[2979]: eglGetDisplay() failed

gnome-session[2851]: gnome-session-check-accelerated: GLES Helper exited with code 256

gnome-session-c[2980]: Couldn't connect to accessibility bus: Failed to connect to socket /tmp/dbus-4UdC671MtT: Connection refused

gnome-session[2851]: X Error of failed request:  BadValue (integer parameter out of range for operation)

gnome-session[2851]:   Major opcode of failed request:  154 (GLX)

gnome-session[2851]:   Minor opcode of failed request:  3 (X_GLXCreateContext)

gnome-session[2851]:   Value in failed request:  0x0

gnome-session[2851]:   Serial number of failed request:  19

gnome-session[2851]:   Current serial number in output stream:  20

gnome-session[2851]: gnome-session-check-accelerated: GL Helper exited with code 256

The solution was found in the outline in the link below, to remove the nvidia drivers.

I was surprised this was the solution as my system ran on intel integrated gpu. However its hard to know how things precipitate till they do.

https://www.osso.nl/blog/ubuntu-bionic-crashing-gdm-eglgetdisplay/

First I changed the display manager to lightdm.

$ sudo apt-get install lightdm

$ sudo dpkg-reconfigure lightdm

to select lightdm as the default desktop manager

Then the login screen came up. However , I entered the famous ubuntu login loop , where upon successful login to the system resulted into the same greeter screen again instead of navigating into the desktop.

$ dpkg -l | grep nvidia 

$ sudo apt-get remove --purge nvidia-***

Voila, the system starts again as before.

I also had issues with the gnome themes, which cause the menus and window title bars to disappear. I had to install the gnome "tweaks" program to select the right theme and things were back.