Why won't my Xbox 360 display?

I use an Xbox 360 S model on an old Asus monitor. It worked fine for a long time before one day it wouldn't display. The monitor says HDMI NO SIGNAL. It only has the one HDMI port. I will note that while turning on the controller turns on the console, it flashes a green circle instead of connecting as player 1. Nothing seemingly happened to cause this. Any ideas?