Check power cable of monitor & VGA cable which connect to cabinet.
Any graphic card in your pc, if yes then confirm monitor VGA cable connect to cabinet through your graphic card (because if VGA cable not connect through Graphic Card, then not show any visual on screen). Check RAM which properly fit in MB or not.
If you check these things then your monitor connect to other cabinet or PC.
If same problem then probably problem in your monitor, check monitor in HP service center.