On Sat, 2 Apr 2022 at 23:21, home user <mattisonw@comcast.net> wrote:
(f34; gnome (is that relevant?), NVIDIA Corporation GK106 [GeForce GTX
660], Dell U2711)

This is a dual-monitor home workstation.

This afternoon, the left monitor suddenly started displaying only dim
vertical stripes and lines and a black background, nothing else.  The
right monitor still works fine.  It is the same when I switch the
monitor cables on the back of the tower, at the graphics card outlets. 

I assume that by "the same" you mean that the left monitor display 
doesn't change.   This means either a cable or monitor issue. 
 

How do I determine whether the problem is the monitor, the graphics
card, the graphics driver, or something else?

Is there something to look for in a log file?  If yes, what do I look
for, and which log file should I look in?

Try swapping the cables and connecting the left monitor to a different
computer.  Modern hardware is so reproducible that the same failure
often shows up across many monitors of the same model and 
similar manufacturing dates.  An internet search may be helpful --
sometimes the fix is known (but may require a professional 
soldering setup to replace the failed component).
 
--
George N. White III