pc monitor wont detect unless xbox gets turned on aswell

I was watching a film on my Computer fine for about 30 minutes on my dual monitor setup: 1x VGA monitor and 1x HDMI monitor/TV.
All of a sudden, with no sudden change in the room and with only the movie playing on the computer the HDMI screen displayed "no signal" and Windows automatically disabled the HDMI screen. when investigating in the screen resolution control panel, the HDMI screen was not detected.

After about half an hour of troubleshooting (Unplugging and changing cables and trying internal graphics (which did not work at all, but that was probably just bios setup)). After trying different cables and trying to "turn it off and on again" I tried to turn on my Xbox 360, to test the TV's HDMI ports, and while the xbox was turned on, I switched back to the PC to find 2 working displays as if nothing was wrong.

I was hoping that someone here would be able to shed some light on why this has happened?

Try switching which port goes to which device. Hell, anything could work... Shouldn't matter, but then, it shouldn't have been a problem.