Jota

Hi. Like the title says I start my sons PC and the monitor says 'going to sleep' and wont display anything. This just started happening a couple of days ago and was fine before, no problems. If I press the menu entry button on the monitor I get the same sleep message popping up before the screen goes black again. Now the strange bit; I try his monitor on my PC and it works perfectly. I try my monitor on his PC and it works perfectly. I put his monitor back on his PC and it's the 'going to sleep' message all over again. What on earth could cause this given his monitor worked perfectly before and works on mine while my monitor works perfectly on his indicating no other hardware issues. (and no, I'm not lending him my gaming monitor and using his older model LCD!) TIA