Thought I would share a scary experience that I went thru in the last couple of days.
I am not a systemd guru, so I was scared out of my wits when this happened to me, even though I never modified my system!
When I booted, I got a message that included the following:
... (preample comments and guidance) ...
... failed to start load systemd_modules_load.service
... (offer to enter maintenance mode or perform other task) ...
I attempted the reboot 4-5 times, each time trying the maintenance mode to dig around a bit and to review the logs with journalctl -xb as was suggested, but I wasn’t spotting the issue or not grasping what I was dealing with. 
After booting from my alternate test disk (xubuntu), I used GParted to perform an fsck on my proper ROOT partition on my main drive, but that did not show any repair action.
I nonetheless attempted a boot from the main drive and still got that boot failure. It again offered maintenance mode, so I entered that and did the fsck using a manual command-line form, and still got not indication of error correction.
I let my instincts about tasks performed during other test installs guide me and I did the following:
sudo update-initramfs
sudo update-grub
# restored from backup
bkpIMAGE="/site/DB006_F8/DB001_F1"
restoreROOT="/site/DB001_F1" # which is / on main OS disk when it boots correctly
( cd ${bkpIMAGE} ; sudo tar cf - etc usr/bin | ( cd ${restoreROOT} ; sudo tar xvpf - ) )
sudo sync
sudo reboot
That, thankfully, gave my primary system back in functional state.
Don’t know if there was a simpler more elegant way of doing it, but that did it for me.
Hope it helps someone else who faces the same situation.
