If you cleaned up something sitting in the root folder of the Macintosh HD, this will happen. Also will happen if you rename a key file, or something gets corrupted.
Your only choice is to boot into FW target mode if you can ... or boot using an external drive.
Boot into FW target, or using an external drive. (HINT: you have an iPod in the sig) and save all your files via network transfer to the G5.
Or will the machine boot off USB, can't remember.