oh...
Monitor was broken the other day - not powering on. Re-plugged the HDMI, wiggled the USB-C adapter, the whole bit.
I'd forgotten to turn the plug on at the wall. Of course, being a laptop, that fired up instantly off the battery, and thus I was led up the "well, obviously I've plugged it in" path. Wasted a good 10 mins before my flakey battery informed me that power was running low....
Been doing IT professionally since 1987, fyi.