Why does the display come up, then suddenly go black?

When I connect the monitor to the HP 6000 (running Win7 Pro 64 Bit) then turn the monitor on, the screen displays properly for 1 to two seconds then goes blank. If I turn the monitor off and back on, it does the same. If I turn on a disconnected monitor, then connect, it does the same i.e: displays properly for 1-2 seconds then goes blank.