Could be the hard drive is defective; or perhaps a cable may contribute to an failed state.
An Apple Hardware Test could be performed (detailed instructions for older mac with
OS X install restore disc can be found online, or in older archived instruction) to check.
And an externally enclosed hard drive (enclosure would need boot-capable chipset to
run the Mac) and formatted for HFS+ is a place to start. The enclosure should be self-
powered with its own external power source; you could run the Mac from a Clone.
So assume the original HDD and/or cable has failed and would need replaced, to test.
If you already have a suitable prepared hard drive with FireWire connections and its
own power supply, with almost any period Mac OS X clone, that may be worth trying.
Ones specific to PPC Mac are necessary, because a clone of Intel-based Mac won't work.
And the hardware for use of PPC boot-capable clone has by nature a niche for just this.
An optical combo drive (internal) or external combo/superdrive with 'oxford chipset'
is another possibility to try & boot a correct system install DVD and see if that locates
the hard drive. A failed hard disk drive &/or bad cable could exhibit what you've found.
You may be able to find suitable external enclosures for use with hard drives from such
places as macsales/owc that can be used to run older Mac OS X via clone/copy. This
kind of active backup can be helpful to troubleshoot older Macs with PowerPC hardware.
Certain archives on how to do these vintage tests & repairs can still be found online. As
I have several older Macs, and no time to re-consider what I'd done decades ago, I'll leave
this at that.
Both hardware and software needs to be troubleshooted. To include known-good replacement.