No discs? Then you have to lean on your backup and restore skills. There is no magic to make a machine start working when it loses a drive or OS. The G5 is pretty old so along the way you must have thought of and made backups and more.
If the drive has failed, some get upset at the obvious but here goes. Replace the failed drive, restore from backups.
Bought a refurbished G5 when mine died but need it's hard drive for work as I use photoshop. Worked fine for months but now programme stalled and since cannot access drive - spinning logo and gradually louder noise til switches off ie wont boot. Tried PRAM, safe start, etc but do not. I do not have discs. 2nd drive which came with G5 boots but I do not have anything on it yet asused old drive.