I had to fix a do-distro-upgrade from 16.04 to 18.04 due to a severed ssh connection, and no screen running (apparently earlier distro upgrades used screen to prevent this kind of problem)
the machine as a PCengine apu2, so no video. also, the root file system is sitting on a miniPCI ssd.
eventually, my thinkpad x230i, and this chroot cheatsheet helped: https://aaronbonner.io/post/21103731114/chroot-into-a-broken-linux-install
mount the root filesystem device
$ mount -t ext4 /dev/<device> /mnt/
if there’s a different boot partition or anything else
$ mount -t ext2 /dev/<device> /mnt/boot
special devices mounts
$ mount -t proc none /mnt/proc
$ mount -o bind /dev /mnt/dev
$ mount -o bind /sys /mnt/sys
chroot
$ chroot /mnt /bin/bash
$ source /etc/profile
In order to help troubleshoot in the future, i followed this advice to get a systemd service unit for a constant shell on the serial port, but mine runs for some reason on S0:
http://0pointer.de/blog/projects/serial-console.html
# systemctl enable serial-getty@ttyS0.service # systemctl start serial-getty@ttyS0.service
It won’t help if systemd does not start, but otherwise it is online really early.