At the cleaning up step of my upgrade to 19.04 from 18.10, the following error was reported:
Could not install 'linux-image-4.18.0-16-generic' installed linux-image-4.18.0-16-generic package post-removal script subprocess returned error exit status 1
Error during commit A problem occurred during the clean-up. Please see the message below for more information. installArchives() failed
Following this, the next option was "Restart", which I did and then arrived at a TTY login prompt. I have logged in to TTY and installed all available updates. If I try to launch Nautilus from the TTY prompt it reports
Unable to init server: Could not connect: Connection refused with a Gtk warning
..cannot open display:.
I tried Boot-Repair-Disk but this only succeeded in removing the nice purple GRUB boot menu, so now I have to boot via the GRUB command line. Using a live USB, I compared the /boot folder with /boot on a good 19.04 machine and all files and subfolders appeared to match with the exception of an efi folder which was missing on the bad machine. I copied the efi folder across from the good machine but this did not help.
Is there anything else I can do to save this system?
Update 21 Apr 2019:
I found and used the startx command which successfully launches a GUI from TTY, albeit with Adwaita, not the Yaru theme and icons.
GRUB is now fixed after I observed /boot/grub/grub.cfg.new and /etc/default/grub.d had been purged and not repopulated at some stage during or since the upgrade. However it is still booting to TTY.
I got Yaru back by installing gnome-tweaks and selecting it with that, and after a few more reboots I am becoming more comfortable with the TTY login. Ubuntu Dock is missing or not working but I will post that separately (although I am starting to like the "Fedora" look of the dock).
Update: After posting the above update I installed lightdm. I am now booting directly into the GUI and no longer needing to log in via TTY and launch the GUI with the startx command.