I have just built my new PC Gigabyte GA-K9NXP-SLI Gigabyte GV-3D1 AMD FX55 2Gb Corsair XMS3200PT 2 x Western Digital 160Gb SATA Drives Antec true control 550W psu everything was working fine until yesterday, I shut the pc down as i was going out. when i came back i switched it back on and the fans started turning then just cut out. i switched it all back off and tried again and the same thing happened. I tried clearing the CMOS ut no luck. i have diconnected everything and i'm still unsure of what it is.
Just connect the bare minimum to see if it works. Also if you can, remove the mobo out of the case, as the grounding pillars maybe causing a short to some of the tracks on the mobo (worth checking) Mobo/CPU/1 memory module/PSU/monitor - disconnect everything else. Then have a look at the power switch, to make sure that is working correctly and is plugged in the correct orientation. If you have another PSU, then I would try that aswell as the 'latch' circuit may not be working correctly within it.