Sounds like the only thing you haven't changed is the hard drive , the video card and the monitor. So lets start by eliminating the monitor first Have you tested it on another machine to know it works ? Next the video card or do you have on-board video? Finally the hard drive . Did you reinstall the OS because with all the hardware changes that system wont work, but if the screen wont come on you havent. So next what post beeps are you getting when you start up? Are these new parts that your installing or second hand parts from other machines? If you answer these questions we can better help you find a solution