I use Ubuntu on an old spare computer, mostly as a low-capability backup in case my usual computer is inaccessible (updates, etc). I had been using Ubuntu 18, but a software update caused a number of strange issues.
I decided that it would be simpler to recover the small number of important files and update to the newest version, 19.04, rather than trying to fix the problem. I installed from a USB LiveCD with no apparent issues, and booted in normally. The automatic software updater suggested some updates, and I agreed.
When I rebooted the system, at the software updater's request, it would not progress past the screen with the Ubuntu logo and the five orange/white dots. The dots would cycle indefinitely, but showed no indication of progressing. I could not use the keyboard at this time; Attempting to press F12 would not open up the terminal that displays the ongoing processes, and CTRL + ALT + F3 or etc would not change to a separate TTY. However, I successfully managed to reboot using Alt + Sysrq + REISUB, meaning the keyboard was apparently recognized in some capacity.
After some experimentation, I discovered that there is a period shortly after the loading screen appears where I can press F12 and have it take me to the console that shows the boot processes. To my surprise, after a few seconds, the login screen appeared, and I logged in with no issue.
After some testing, I have determined that if I leave the computer to boot normally, it will consistently hang on the screen, but if I change to the console before the keyboard is ignored, it will consistently succeed in loading the OS. I would like to know why this is, and ideally how I might be able to fix it.
First update: I attempted to start the OS "normally", and after some time pressed F12. This time, it brought me to the terminal, with a message
A start job is running for Hold until boot process finishes up (Xmin Xs/no limit) at the bottom of the list. I have seen this message a few times; It was something that occurred even before I installed the new version. Looking up this phrase has yielded some questions on various sites, but I have not been able to use their answers in my case.
When I tried to reboot and reach the login screen by pressing F12 near the beginning, as before, it did not succeed; Evidently I had not been fast enough. My keypresses would not be accepted.
A third login attempt allowed me to get in by pressing F12 early.
Second update: After my previous successful login (where I managed to get in by hitting F12), the computer took an excessive amount of time to shut down. I ultimately ended up forcing the power off by holding the power button. I suspect this is related.