Another set of updates, another recurrence of "Low Graphics Mode". This time
resulted in '/etc/X11/xorg.conf not found'.
sudo rm -v /etc/X11/xorg.conf
Entered but wound up back in 'Low Graphics Mode' after the reboot.
BTW, attempting to use the 'One Time Boot Into Low Graphics Mode' option just drops to console. Using startx just returns 'xserver already in use' (or similar) message.
Finally used Ctrl-Alt-SysRq+REISUB which finally got booted correctly.
Is this a flaw in the kernel module for Nvidia? I seem to get 'Low Graphics' Mode' about once a week; more often after various updates.
EDIT: Latest updates seem to have solved this for now *fingers crossed*