I just upgraded an Ubuntu server from 16.04 to 18.04. All went seemingly smooth, but after a restart I'm not able to log in.
The server boots, I can see all the usual logging output on the screen, some services start properly, but it never arrives at the console login prompt (no X on this machine).
I can access websites served by the machine, so at least nginx and postgresql services have started, but – unfortunately – not sshd.
I know it's all a bit fluffy, but what do I do to regain access to my server, either via SSH or a local console at the physical machine (or both, preferably)?