Are you getting post beeps on startup ? If not you may be dealing with a power issue . Just because your fans are running and your motherboard light comes on doesn't mean you have enough power in the power supply to run the machine .Power supplies can die slowly and still appear ok .If you have access to another power supply try changing it out . You may also want to try a different monitor or test your old monitor on another computer. depending on your motherboard you could try resetting the mem button common on Asus boards. Also you mentioned you've disconnected the power and that you've held down the power button for extended periods . Did you do this together? if not try unplugging from power and holding the power button down for roughly 60sec then plugging in and rebooting. Finally as dumb as this is going to sound but it happens to the best of us... is your monitor turned on and plugged in? Just sayin Hope something here helps and good luck