Dead motherboard battery (probably not as that would cause a loss of CAMOS settings but you should still get something on boot attempt), Power supply (could be but check connection to motherboard and all hardware), Motherboard, video card, dead monitor. Of these problems, I have had the: dead motherboard battery, dead power supply, dead video card, and dead monitor. It is likely something simple and cheap but could cost a bundle solving the problem when you can not identify the problem.
