Why does my newly built computer start up then shut down almost immediately?

[Crush]

I just built my very first computer and I assembled everything outside of the case, it worked fine. Then I assembled everything inside the case and with poor cable management it worked, all the fans started to spin expect for the front fan so I tried to plug it into some cable, not sure which one it was one connected to the PSU. After I tried to do that I again tried to power on the computer (from the power button on the front) and it turned on for just about half a second, the LEDs came on then it all shut off. I tried to turn it back on from the front of the case but it didn't work, so I unplugged the PSU and turned it off then waited a minute or two and tried again, same thing happened but faster, on the off. It's like a flash of light almost. So I was really worried and I went back into the store I bought it from and the guy said it was probably the CPU Heat Sink fan that wasn't secured right, or any of the power cables to the motherboard. It wasn't either of those because I made sure the power cables were plugged in and I took the heat sink fan out and put it back in securely and it still doesn't work, he said it wasn't the PSU or the motherboard that was broken, I just bought it the day before. Anyone know what else it could be? I just don't want to have to take in the whole computer tomorrow. If you could help that would be great!

Computer Specs:
CPU- Intel i5-3570k LGA1155
GPU- GeForce GTX 650
MoBo- Gigabyte Z77-DS3H
HDD- 3.5" WD Black 1TB
PSU- Corsair CX600M Modular (80 plus bronze)

Anything you guys can help me with? I'm pretty familiar with the computer all where everything plugs in because I've done research the only thing I'm not too good with is the HDD and all the SATA plugs and the 3 cable plug in things but I think I've done those right because it did work.

PS: I didn't get to the bios cause I wanted to fix the fan at the front of the case.
Forgot when I was installing motherboard there were 7 standoff holes and the 7th didn't line up to any on the case so I just ignored it and was careful plugging anything in. Could that have to do with anything?