Does the question mark eventually disappear, or does the G5 remain in that state?
If you can't get past the question mark, then reinstall the operating system (while retaining all the other items on the hard drive).
If the question mark disappears after a while and the G5 then boots, you could try to speed up the G5. Download the free utility AppleJack from
This will run various tests, dump unwanted cache files and correct some problems. Just start up the G5 with Command-S pressed, type
applejack auto restart
and then press the Return key.
It's a good idea to run AppleJack regularly – even on a Mac that appears to be working perfectly – as it can deal with problems before they cause noticeable trouble.
Another useful program for keeping the Mac in trim is Macaroni. It will run automatically to carry out daily, weekly and monthly maintenance tasks. After installation, it is recommended to "Run Job Now" to "Remove Localized Files". This typically removes an astonishing number of unwanted foreign-language files. Over 50,000 files removed is not uncommon, and the disc space saved may be more than 1 GB.
Removing unnecessary files should speed up operations, but defragmentation may further improve matters. Disc maintenance software is available to do this, or you could use backup software to backup your hard drive and then use the same software to reinstate the files on the hard drive. This will result in the files being stored in an orderly fashion, rather than as fragments scattered over various parts of the hard drive.