Have you tried a window system restore?
if that fails you wouldn't happen to have another monitor or TV you could try out?
and if that fails then I'm thinking maybe RMA time
Have you tried a different connection, like DVI for example, instead of just different HDMI cables?
He is using the VGA-port which is also placed at the same position as the HDMI, isn't it?
Maybe one of the contacts has been damaged? Have you tried using the display-port slot if you got one? Cause normally it should recognize that there is a device at the HDMI port if it does at VGA. So try another screen of possible (Maybe TV) at the HDMI port, if it happens again it could be the port itsself.
The other possibility, as sgtbilko already said, would be to try restoring your system to the day before (but to be honest i don't have much restore points as many other people).
Maybe also a win8 update caused that, they often have problem with drivers, have you tried to use an older version of your graphic cards driver?
have a nice day,