Most Computers will check the CD tray for bootable media FIRST before the harddrive. Even though you don't SEE that happen.
Pop the disc in, and try to power on, see what happens.
If nothing happens......you may have to remove the harddrive, and attach it to a different computer to repair it.
But, if you are not able to even get into BIOS, (assuming you are not bypassing BIOS, by changing the settings in BIOS) you will need to replace hardware of some sort.....could be RAM, VIDEO processor, Main CPU, MOBO.
If you get absolutely NOTHING on screen when you power on.....andi mean NOTHING, no blinking cursors, no logos, NOTHING. I mean nothing like, he screen looks the same when its powered off, as it does when you power it on.....then you have dead hardware, plain and simple.