second monitor suddenly not detected (resend)

[Colin Law]

On Sat, 26 Sep 2020 at 06:02, Karl Auer <kauer at biplane.com.au> wrote:
> ...
I tried switching from Ubuntu to
> Gnome and back at login, no change.

Do you mean switching from Wayland to Gnome?  If so did it work in
Gnome?  You may have meant to imply that but didn't say so explicitly.

 Tried update/upgrade and it
> upgraded a surprising number of packages in including Nvidia libraries
> and suchlike, but no change. Ran "sudo dpkg-reconfigure gdm3", no
> output and no change.
>
> Short of re-installing Ubuntu, is it possible to reinstall or
> reconfigure the graphics subsystem? This is a Lenovo Y540 with Nvidia
> graphics hardware, and currently the nouveau driver is active.

What happens if you log on as a different user?  Create one if necessary.

Colin