Hello,
my computer was running normally, then I shut it down for the night. The following morning it wouldn’t boot. When I press the power button, I can see the power and disk LEDs light up, fans start spinning and drives spin up too – and that’s all that happens – the monitor doesn’t even come up. I got a PC speaker out of an old computer and connected it to this one, but there is not a single beep when I start the thing. It simply just sits there hanging with no visible or audible error message.
I opened the case and took everything out to get rid of dust. Then I plugged the CPU and one memory stick into the motherboard, connected my monitor into the on-motherboard VGA port, but still the same. Resetting the CMOS didn’t help and neither did changing the battery. I tried with a different PSU, but to no avail. I also tried putting different RAM sticks into different slots when trying to get POST in the stripped-down configuration.
I didn’t notice any warning signs before this happened. The computer ran normally with no stability issues – it simply stopped working, just like that.
I’m suspecting the motherboard, myself. It’s rather old-ish (I bought it about two and a half years ago) – an Asus M2NPV-MX. It could also be the CPU, but I doubt that as the CPU is new (I bought a new one recently, about four months ago, after my old one failed – L2 cache failure, to be precise). I’m not sure it’s the motherboard, though – and I’d like to get some insight before I go and buy parts. 
The box is an AMD Athlon X2 5400+ on an Asus M2NPV-MX motherboard, with two 512 MB and two 1 GB sticks of Kingston DDR2 RAM.