I am using Ubuntu 20.04, a fresh install.
I installed all of the latest updates, and now when the computer is starting up, it shows a black screen with the mouse visible, but is entirely unresponsive. The only thing that could have affected it was that I installed the linuxwacom driver (that maps buttons to Xinput events) and then shut down.
Starting back up, it does exactly what I described above.
I can access the motherboard’s BIOS. (By holding the power button for 4 seconds)