well the fact that he can hear it indicates teh game is playing blind. It definitely has something to do with the monitor. It could either be a bad connection to the monitor, possibly loose/cut/missing wires, or a bad monitor itself.
Since he says it's happened before, and he coudl "jiggle" it to work... I'd look for loose connections first. Try to reseat anything you can and just generally check the wires and connections. Otherwise, maybe that "jiggle" was just luck and the monitor actually went out this time... I'm far from a pro, but these are the first things I think of.