Ubuntu 20.04.2.0 LTS desktop installation:
Following guidance from this post's accepted answer:
How to restart X Window Server from command line?
I ran:
sudo systemctl restart display-manager
(it may be useful to know that the tab autocomplete suggested adding .service to the end, but I did not do so)
My screen went black and unresponsive, perhaps I should have been more patient but after about a minute I decided it wasn't coming back on its own and hit the hardware reset button on my PC. Since then, when booting into my Ubuntu installation I am met with a completely black screen with no cursor. Attempting to log in to a TTY is completely ineffective, my only recourse so far is to restart into BIOS.
I have managed to get the installation media running with the drive holding my inaccessible OS mounted. A comment in the linked post's accepted answer had the same problem and indicated that the solution involved restarting the service from terminal. Naively attempting that approach from the installation media (the commenter did not provide a command), running the command again from the installation drive:
sudo systemctl restart display-manager
simply restarts the display manager in the installation environment (but does so in this case without bricking anything else and actually coming back). The OS still boots into a black screen.
I do not wish to re-install in any capacity if I can avoid it.
Additional information:
Output of cat etc/systemd/system/display-manager.service
[Unit]
Description=GNOME Display Manager
replaces the getty
Conflicts=getty@tty1.service
After=getty@tty1.service
replaces plymouth-quit since it quits plymouth on its own
Conflicts=plymouth-quit.service
After=plymouth-quit.service
Needs all the dependencies of the services it's replacing
pulled from getty@.service and plymouth-quit.service
(except for plymouth-quit-wait.service since it waits until
plymouth is quit, which we do)
After=rc-local.service plymouth-start.service systemd-user-sessions.service
GDM takes responsibility for stopping plymouth, so if it fails
for any reason, make sure plymouth still stops
OnFailure=plymouth-quit.service
[Service]
ExecStartPre=/usr/share/gdm/generate-config
ExecStart=/usr/sbin/gdm3
KillMode=mixed
Restart=always
RestartSec=1s
IgnoreSIGPIPE=no
BusName=org.gnome.DisplayManager
StandardOutput=syslog
StandardError=inherit
EnvironmentFile=-/etc/default/locale
ExecReload=/usr/share/gdm/generate-config
ExecReload=/bin/kill -SIGHUP $MAINPID
KeyringMode=shared
ExecStartPre=/usr/lib/gdm3/gdm-wait-for-drm
Edit:
After fiddling around in recovery mode, I am able to boot into the system’s TTY if and only if GDM3 is not the active display manager and normal boot is performed through the recovery mode menu (the “resume” option). Despite reinstalling ubuntu-desktop and a new display manager the best I can do is get into TTY through recovery mode.
