If the hard disk drive were formatted correctly for use in that computer, and
is of the correct hardware type, the installation of a supported OS X version
would usually require the install-restore DVD media. The tools on that disc
are very helpful in preparation, installation & troubleshooting of the system.
this isn't the one I'd wanted to post, the Support article for reformat is gone
{There was a youtube that suggested it would show how to do all that stuff
but when viewed, it seemed to only show the computer as a centerpoint of
a walk-around, most of 17 minutes like it was expected to be self-animated.}
The correct format and hard drive type, is just a start. Certain models of these
G5 towers had specific issues or probable failures that may need attention.
If the computer could be booted in Target Disk mode, a second Mac may be
able to see it as an external drive, then disk utilities from that running one
may be able to attend to preparation for installation from a correct disk image.
However the disc image would have to be exactly for the PowerPC version OS
and not one taken from an Intel-version of similar-name Leopard OS X.
Since I'm unfamiliar with the unit enough to guess where details are lacking,
these are the issues and areas that I'd need to know first, if it were mine. To
have a retail version of Leopard OS X 10.5, if not the original system DVD,
would be a place to start. Usually a working optical drive is first, if nothing else
in the hardware is damaged. And a new PRAM battery on the logic board.
Good luck in this matter! 🙂