Cant turn on my pc

I built up this rig 2 weeks ago, it was running perfectly and i even did some minor oc and have it running 24/7 stable for 2 weeks.

Yesterday i got a new monitor, set it up and everything run fine. Then i shut my pc down, open up the casing to show my friend some inside look of the design of my casing.

I unplug the 24pins cable which was blocking the view, and then plug it back
but now the pc wont boot.

When i press the power button, it on for 0.5sec and turn off again, 3sec later, on for 0.5sec and off again, and the on/off loop will occurs until i turn off my psu.

I have done the following to try to figure out the problem.
1. reset cmos with jumper, same on and off loop
2. unplug ram, same on and off loop
3. unplug ram/gpu, same on and off loop
4. Unplug the 24pins cable, with all other (hdd, gpu, cpu) cable intact. short the no.15 and no.16 hole of the 24pins cable with a pin, the psu will start up and the psu fan and other casing fan which is connected to psu straight, will spin as long as im connecting the 15th n 16th hole. I know the hdd is spinning as well.

Is my mobo dead? Thanks in advance.

My rig:
DFI LP UT x48-t2r
intel Q9550 E0 @ 3400mhz
Corsair Dominator twinx4096-8500c5df 5-5-5-15
PC power & cooling 750 watt quad
Palit HD4870 sonic dual
640g+500g Seagate
LG Dvd burner
Creative X-fi Fata1ity Xtreme Gamer
Lian-li V1110B midtower casing
Vista x64 Ultimate

That sounds like a power button that is stuck in bridged (closed) mode.

Try removing front panel wires and manually starting the mohterboard. Onboard switch if it has one, or jump the power button pins with a flat head screw driver.

my board have an on board switch. power on using it yield the same result.

i have tried to use another 450w power supply, plug in the 8pins + 24pins, power on, and same problem.

Seems like its my mobo problem?

My pc exhibits similar behavior. When it flakes out like that I have to flip the switch on my psu, let it sit for 3 or 4 minutes, then flip the switch, push the on button.. and rock and roll... only happens on a hot reboot that actually causes a power cycle. Windows restart doesn't cause it.. but the reset button does....

i face the problem lastnight, couldnt fix it and i then unplug the psu powercord entirely before i went sleeping last night.
tried this morning again but it aint fixing.

To make it clearer, once i hit the power on button once
it will turn on for 0.5sec, off itself, silence for 3sec, and AUTO on itself again for 0.5sec and off, and keep repeating without my intervention.
Only hit the power on once and it will on/off loop infinitely.

does you psu have a switch?

yes, i turn it off and unplug the powercord last night before i gone bed.
Same problem today.

you sure that you have connected the power pin correctly to your motherboard