My homebuilt system that I have been using for a year without any problem suddenly crashed after turning it on after being away from home for two weeks. It booted up fine but when I allowed it to update Java, the screen went black and everything froze. I tried to reboot at that point but the power and reboot switches didn’t work. I pulled the power plug and shut everything down, but have not been able to boot up the computer ever since.

Power seems to be getting to the motherboard since the power light on the motherboard turns on when I flip the power switch behind the power supply; but the PC won’t post. I even tried switching power supplies just to be sure, but with the same result. I also tried booting the computer by touching the prongs on the motherboard that correspond to the power button with a screwdriver, but again with the same result. Nothing on my system is overclocked.

Does anyone have any idea what the problem could be? Could my CPU or motherboard suddenly have gone bad? What is my next step?

My equipment is as follows:
Motherboard: Asus A8N32-SLI Deluxe
CPU: Opteron 165
Power Supply: Antec NeoHE 550
HD: WD Raptor 150GB
Memory: Corsair XMS-3500LL PRO

Thank you for any advice.

The best way is to strip it down to a barebone on a cardboard or the mobo box. It'll be just the mobo, cpu, ram, gpu, psu, keyboard & monitor. No drives. No external devices. Clear cmos for 10 mins. Your manual explains this. Try each stick of ram in each slot.

http://www.ixbt.com/mainboard/asus [...] ard-sm.jpg

Yes, could just also be as simple as something needs to be pulled out and reseated, like your memory, maybe a loose plugin from your power supply. When I moved last year at first plugin, my PC would not do anything. I didn't have time to mess with it at the time, and left it setting there only to come home a few hours later to see my wife setting there with it running fine. I asked here what she did, and she said " I just took the side off the case, wiggled some the wires and cards around a bit, and it turned on perfectly". I had to smile, but it's been fine since.
Could be something else, but start at the simple basic things first and work you way up.

This exact same problem happened to me a few months ago.

It was my power supply, which after years of use was full of dust. The power light would come on but it would not post. A simple change of my PSU and everything was fine.