iMac G4 will not boot

My aunt has a G4 iMac "running" Os X 10.2.5. When we turn on this computer, all we see is a grey screen with a disk icon in the center of the screen and a flashing question mark on the disk icon. I asked her for the disks that came with the computer so I could troubleshoot the disk, but she lost them. I tried to run a version of Debian Live Linux CD to see if I could do something with the hard drive, but I could not get it to run. I tried all the key commands at startup, but nothing will work. I hope somebody can help