I upgraded from Ubuntu 18.04 to 18.10 without any errors. When I restarted, I got a black screen with a blinking cursor (couldn't type anything) right after GRUB. Pressing Ctrl-Alt-F2 got me to a terminal session.
I rebooted, then tried editing GRUB by pressing
e and changing
quiet splash to
nomodeset. Pressed F10. It didn't work.
I had to reboot, press Ctrl-Alt-F2 then remove the nvidia drivers by typing
sudo apt-get purge ^nvidia. Only then was Ubuntu able to boot successfully.
I installed the nvidia 340.107 drivers from the Additional Drivers tab in Software & Updates. Rebooting got me to the same black screen as before, and I had to purge the nvidia drivers again.
System: AMD FX-6300 CPU, Asrock 970 Extreme mobo, 8 GB RAM, Nvidia GeForce 9600GT.