Comment # 1 on bug 901887 from Lars Marowsky-Bree

I had played with xrandr --off / --auto for a while for that output (turning
the monitor's power on/off etc as well), and then eventually gave up and turned
the display off.

After ~2h hours, I decided to give it another try, turned on the monitor, and
ran xrandr --output DP4 --auto again. (A sequence I had done multiple times
before). Lo and behold, this time, the monitor came online fine and is now
displaying.

I don't know why, and I don't really trust the result ;-) It was also not
reproducible, there's nothing new in the logs from either X or in dmesg. I
can't tell what is different now.