Mac Pro w/Leopard - Can't find Hard Drive

When I turn on the MAC, the grey screen appears and after a while, a folder with a question mark comes up. I tried restarting while holding down C Ley for Snow Leopard DVD and for Leopard DVD. Both DVDs try to install instead of opening my home screen. I said fine, I will reinstall. When I get to the screen that says to choose which disk, there is nothing there, just blank white space. Then when I back track, I try to choose a start-up, but all I have is the DVD and an icon for a hard drive that says search network.

I have gone through the PRAM, holding down the Shift Key and holding down the Option Key. None of them work except for the Option Key, I can see and move my pointer but that's it. I did not have a system back-up because I figured if I have to reinstall using my DVDs, it would not be a problem.

I don't know how or why my hard drive has disappeared. Has anyone been through this?