If your optical disc drive tray is open when your Power Mac G5 goes to sleep, it will retract the tray as it goes to sleep. Then when the computer wakes up, the disc may appear in the Finder as blank media.
This can occur with blank CDs or DVDs as well as commercially-produced CDs or DVDs.
If this occurs, eject the disc icon on your desktop by dragging it to the Trash in the dock. Reinsert the disc and close the drive tray. The optical drive will recognize the media correctly.