xrandr doesn't detect HDMI monitor

Question

My second monitor has suddenly stopped working after a reboot.

I'm on Ubuntu 18.04 with an OpenBox desktop and an nVidia 1060, using the nVidia 396 driver. I've been fiddling with my display settings a bit because of a different problem and after a reboot, the second monitor says "no signal", and my double-wide desktop has shrunk to a single monitor. I have no idea what I've done!

xrandr reports only one monitor (HDMI-0) despite HDMI-1 also being plugged in:

$ xrandr 
Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 32767 x 32767
DVI-D-0 disconnected primary (normal left inverted right x axis y axis)
HDMI-0 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 477mm x 268mm
   1920x1080     59.93*+  60.00    59.94    50.00    60.05    60.00    50.04  
   1680x1050     59.95  
   1600x1200     60.00  
   1440x900      59.89  
   1440x576      50.00  
   1400x1050     59.98  
   1280x1024     75.02    60.02  
   1280x960      60.00  
   1280x720      59.94    50.00  
   1152x864      75.00  
   1024x768      75.03    70.07    60.00  
   800x600       75.00    72.19    60.32    56.25  
   720x576       50.00  
   720x480       59.94  
   640x480       75.00    72.81    59.94    59.93  
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-0 disconnected (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
DP-3 disconnected (normal left inverted right x axis y axis)

Things I've tried:

lxrandr, arandr, and nvidia-settings all fail to detect it.

I was told xrandr --output HDMI-1 --auto might reactivate it, but it didn't.

Unplugging and replugging the cables made no difference.

Any help appeciated!