About 20 minutes ago, I performed a standard upgrade ("apt-get upgrade"). Upon reboot, I was dropped to an emergency shell, and cannot boot normally. Here is the information from /var/log/apt/history.log:
Start-Date: 2018-02-05 23:47:41 Commandline: apt-get upgrade Upgrade: libsystemd0:amd64 (229-4ubuntu21, 229-4ubuntu21.1), libsystemd0:i386 (229-4ubuntu21, 229-4ubuntu21.1), grub-common:amd64 (2.02~beta2-36ubuntu3.15, 2.02~beta2-36ubuntu3.16), libxatracker2:amd64 (17.0.7-0ubuntu0.16.04.2, 17.2.4-0ubuntu1~16.04.4), grub2-common:amd64 (2.02~beta2-36ubuntu3.15, 2.02~beta2-36ubuntu3.16), udev:amd64 (229-4ubuntu21, 229-4ubuntu21.1), libegl1-mesa:amd64 (17.0.7-0ubuntu0.16.04.2, 17.2.4-0ubuntu1~16.04.4), grub-pc:amd64 (2.02~beta2-36ubuntu3.15, 2.02~beta2-36ubuntu3.16), libudev1:amd64 (229-4ubuntu21, 229-4ubuntu21.1), libgbm1:amd64 (17.0.7-0ubuntu0.16.04.2, 17.2.4-0ubuntu1~16.04.4), grub-pc-bin:amd64 (2.02~beta2-36ubuntu3.15, 2.02~beta2-36ubuntu3.16), systemd-sysv:amd64 (229-4ubuntu21, 229-4ubuntu21.1), libwayland-egl1-mesa:amd64 (17.0.7-0ubuntu0.16.04.2, 17.2.4-0ubuntu1~16.04.4), libpam-systemd:amd64 (229-4ubuntu21, 229-4ubuntu21.1), grub-rescue-pc:amd64 (2.02~beta2-36ubuntu3.15, 2.02~beta2-36ubuntu3.16), systemd:amd64 (229-4ubuntu21, 229-4ubuntu21.1), libmbim-proxy:amd64 (1.12.2-2ubuntu1, 1.14.0-1ubuntu0.16.04.1), libgl1-mesa-dri:amd64 (17.0.7-0ubuntu0.16.04.2, 17.2.4-0ubuntu1~16.04.4), libmbim-glib4:amd64 (1.12.2-2ubuntu1, 1.14.0-1ubuntu0.16.04.1), mesa-va-drivers:amd64 (17.0.7-0ubuntu0.16.04.2, 17.2.4-0ubuntu1~16.04.4) End-Date: 2018-02-05 23:49:30
I believe the mostly likely package at fault would be systemd, since it did (sort of) boot, but I'm quite unsure. This occured on Ubuntu 16.04.3 LTS
Update: Choosing the same kernel but with Upstart next to it in grub succeeded in booting. I am getting systemd-login errors, however. This made the system usable, at least for me, though a lot of Ubuntu users would have been stuck. This should be a priority.