clean Bullseye install, display disappears after "loading initial ramdisk"

[Mark Copper <mlcopper@gmail.com>]

Where to start diagnosing a problem like this?

Minimal install -- no desktop.

No "oops" in syslog though there are some warnings.

System seems to shut down properly when power button pressed.
Yesterday I could even ssh in, but that facility disappeared.

Asrock H570M-ITX/ac with Intel i3-10100. More info gladly provided.

Thanks for reading.