My (actually this) Debian server failed to boot after a power failure last week, it turns out the graphics card failed too, probably because of the cold and the thermal shock, but replacing the card did not allow the computer to boot.
With systemd, if something happens in the boot process, despite some obviously specific failure triggering the problem, it tends to fob you off with a message to run
This feels like the machine equivalent of “Hey, I just saw some needle go past that broke the machine. Let me hand you a haystack so you can find it.”
The command that is a whole lot more useful, is:
systemctl status systemd-modules-load.service
which allows much more rapid diagnosis of many problems. In my case the whole thing turns out to be an old line in /etc/fstab – an obselete line to mount /proc/bus/usb. It shouldn’t have been there anymore, but I am slightly amazed and irritated that the whole boot process was abandoned because of one line in /etc/fstab.
But hopefully someone else in this situation will find the suggestion to check that file first useful.