PC won't turn on, what's the problem?

Question

Power Supply

The computer does not turn on from the button, although the power bulbs inside are lit. CPU Phenom II X4 940
PSU Ascot 620w MB M4A79 Deluxe ASUS Video card NVIDIA GEFORCE GTX 580
After a quiet shutdown, it did not want to turn on, vacuumed the PSU and did not help

Answer 1

Disconnect everything from the motherboard, leaving only the processor - turn it on - listen to the "squeak", there should be one long. If there is - connect a video card - listen to the squeak, it should be short (hmm ... two or three, but it doesn't matter). If there is - stick memory - listen to squeaks - there should be one short - normal start.
No disks or anything else there is superfluous, do not connect, no network cards, no mice with keyboards.
The initial diagnostics is over - the results are here!
Although it does not hurt to check the "manual" (instruction), which describes how and where the diodes should burn ("I and the diode").

Answer 2

The fact that the LED is on does not mean that the power supply is fully functional, so you need to take a working one and try it. Well, or at the very least, disconnect the power cable, press the power button several times, then disconnect all cables from the power supply and reconnect.