This is why I do plenty of backups

I've also had huge problems with my two G5:s. Both died on me. After having them checked, we first thought it was due to faulty RAM becise we could get them to boot again. But they broke down again. It turned out to be a common G5 problem: the computer runs so hot that various chip solderings weakens and fall off. Not break, but get loose. You can apparently fix it by reheating the soldrings with a really hot hair dryer. If you are lucky.