Computer turns on for 1 second then shuts off

[Guest]

I built a custom computer in october of last year.
CPU: Intel Core i7 920
Mobo: Gigabyte EX58-UD3R
RAM: 4GB [2X 2GB] Corsair DDR3
Graphics: EVGA GTX 260 Core 216
PSU: Coolermaster 550W PSU
HDD: Western Digital 500GB & Hitachi 350GB
OS: Windows 7 Home Premium 64-bit

Two days ago while rendering out a sequence in Adobe After Effects the computer shut off suddenly by itself. It was not due to overheating, all recorded temps were normal. I thought it was nothing, turned it back on and went back to normal use. The next day [yesterday] I was playing Bioshock 2, same thing happened, it shut down without explanation, all temps were normal. So, I thought something came loose that was causing this, so I opened it up, nothing was loose. Noticed it was a little dusty, so I took a compressed air can and dusted it out, replaced CPU thermal grease then put everything back together. I turned it on, and the internal fans and LED lights came on for 1 second, then died. I tried a few times to turn it on and nothing happened. I opened it back up, checked everything, reset Bios then tried again. Nothing.

I have no idea what could be wrong. Please can someone help?
[fyi, i know alot about computers so dont hold back when it comes to vocab]

 

[BrentS]

I found this thread searching for why my custom PC was spooling up for a couple seconds and then turning off before any display showed up. This is how I fixed it for me.

I opened up the case and found mounds of dust, so I cleaned it all out and I also noticed my power switch was loosely plugged in as well as the plug from PSU to MOBO. Tightened everything up, cleaned up dust and then turned it on and the MOBO settings came up. I left as is and restart it. No luck still turning off. In the mobo screen it said cpu was 55 °C so i waited a while for it to cool and fired it back up. Success, i got to the mobo setup screen and I restored the default settings. Me thinks the cpu ratio was too high to continue. Now no more overclocking for me until I can upgrade my rig. Its been a great 5 years, I couldn't be happier.