PowerMac G5 not starting up


Fairly new PowerMac G5 worked beautifully. Turned off for the weekend, and returned on Tuesday to a non-booting machine. Get a light gray background with a dark gray apple icon PLUS a rather loud roaring noise coming from the CPU fan. I have not installed anything new. I have unplugged everything except the monitor/keyboard and nothing. I have attempted to go to safe mode, won't do it. I have attempted to use the install disk to run the disk utility, won't do it. I was able to use the apple hardware test and there's nothing indicated on that. I tried resetting the PRAM but never heard the second start up sound, just got a black screen.

Anyone with any ideas or advice - please help! I'm stumped!!! Thanks :(
what helped for me a couple of times (g4 quicksilver) was resetting the motherboard using the little button on the board. but i don't know if a g5 even has that button, and i also remember reading that you should never do this..

if you still wanne try, disconnect all equipment and powersource.
Issue: Upon pressing the power button, our PowerMac G5 (2005) tower would beep twice, then loud fan noises, screen wouldn't come on. Eventually we would just have to power it down. We tried pressing the PMU (or maybe it's called the SMU in this edition??) but to no avail. Couldn't locate the internal power, as some forums suggested, and couldn't turn on the computer to reset the PRAM (also suggested).

We called Apple and they had us unplug the computer and remove any devices plugged into the computer. We then removed the RAM cards, replaced them (we had to push rather hard to get them back in), then plugged everything back in and powered up the computer with four keys pressed -- Option, Apple, "P," and "R." He had us let the keys go when we saw the apple and the spinning gear.

So far, our computer is working. The Apple guy recommended that we shut down the computer every night.