Things like "screen went white" and "keyboard light on but doesn't seem to power up."
It will help if you can describle what happens when switching on from "cold".... any sounds of fans or disks, any lights come on at all except the keyboard's "power" light?
Do you hear any beeps?
Does the monitor stay black or show any messages at all?
A long shot, but worth doing if you are happy to look inside the box, is to make absolutely sure the video card is properly pushed home and the data cable at the back of the computer is well connected.