PC boots and shutdown after 3 secs (Please help me!)

Hi,

I have a problem, yesterday I came back home and I my computer was off. I tried to boot it up, the fans works and some lets but after 3 seconds the computer shutdown. I can't even see monitor to get into BIOS?

This is my computer:
AMD Opteron 165 Denmark 1.8GHz Socket 939 110W Dual-Core Processor Model OSA165CDBOX

Western Digital Caviar SE16 WD2500KS 250GB 7200 RPM SATA 3.0Gb/s Hard Drive - OEM

Thermalright XP-120 CPU Cooling Heatsink - Retail

G.SKILL 2GB (2 x 1GB) 184-Pin DDR SDRAM DDR 500 (PC 4000) Dual Channel Kit Desktop Memory Model F1-4000USU2-2GBHZ - Retail

DFI LANPARTY UT nF4 Ultra-D 939 NVIDIA nForce4 Ultra ATX AMD Motherboard - Retail

XFX PVT84JUDD3 GeForce 8600GT XXX 256MB 128-bit GDDR3 PCI Express x16 SLI Supported Video Card

Rosewill RP550S-2MK 550W ATX 12V Ver.2.01 Modular Modular Power Supply /Black

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Please help me, what happend to my computer? my psu is dead? HELP!!!
HELP MUCH APPRICIATED

Check to make sure the power cord is firmly plugged into the PSU(hey, it happens- they can be loose). Make sure rocker switch is all-the-way in the "on" position.

You say your computer was off when you came home, if there was some kind of power outage, there could have been a voltage spike that damaged your PSU.
Are you plugged into a surge protector, UPS or just a "power strip"?