|Does anything come up on display? Do you get any POST beep?|
Disconnect the harddrive data and power cables as well as cables to any optic drives.
Will it boot up into BIOS and give any display? If not you may have failed graphics, failed CPU, failed motherboard, failed powersupply. There isn't enough information in your description to tell much more at this stage.
Goin' Fishin' (Some day)