PowerMac G5 not booting

Heya--it's probably not a hardware problem, don't panic. The first thing you should do is boot into single-user mode (hold down command+S after the startup chime) and run fcsk. Once that is done, type "exit" and you'll continue booting into OSX as normal. If you end up on that screen again, shut down and try booting from your OSX install disc. If you can boot from that, do an "archive/install", as likely that just means that over time the kernel ended up with an error of some sort (an archive/install won't risk any of your data, it'll just overwrite the OS). Then run the software update and you're gold, but start running regular maintenance scripts if you don't already (I use Cocktail to run most of mine), as our machines are on the older scale these days, so keeping them running in top form is even more important than it used to be...

If you get the screen while trying to boot from the OS install disc, then it might be some kind of problem with the hardware (or firmware). Again don't panic; try resetting the PRAM, and if that doesn't sort it out, I'd try removing RAM pairs, as if it IS a hardware problem, and that's a big if, it's likely either the RAM or the hdd. If it was the hdd, you likely wouldn't make it to the auto-restart screen, so it would more likely be RAM. If it does turn out to be RAM, you're in luck, as RAM for G5s has never been cheaper, so while fixing the problem you could be upgrading at the same time!

Clear and concise with about 3 too many "likely"s for my tastes.

No problem, glad I can help. Let us know how it turns out, yeah?

Are you using a wireless keyboard? Try holding down the mouse button while booting. This may work...not sure. I know it will eject disks on startup.