Thanks, guys, for those suggestions.
CTRL+ALT+F2 may well produce a response from the system, but nothing to see on the screen!
I don't know how to boot the system to runlevel 1.
When I power on, I get a brief flash of the setup screen (containing the option to Press F2 to enter setup). This disappears into blackness after 3 seconds, and the system pauses waiting for my admin password. If I input this normal bootup sounds like it continues but with black screen. If I do press the F2 after powering on, I can see the setup screens. The display lasts about 5 seconds, barely enough to change page or tab through some fields, before that goes black too.