New built computer not turning on.

Hi, I just made this new computer today, and I cant figure out why it wont turn on. This isn't my first computer, I made about 3 computers in the past wich ran fine But for this computer the fan lights up for about 1 second, and everything shuts down again.

here are my specs :

AMD FX-8120 8-Core Black Edition Processor Socket AM3+ - FD8120FRGUBOX

ASUS M5A99X Evo - AM3+ - 990X - SATA 6Gbps and USB 3.0 - ATX DDR3 2133 Motherboards

OCZ ModXStream Pro 700W Modular High Performance Power Supply compatible with Intel Sandy Bridge Core i3 i5 i7 and AMD Phenom

EVGA GeForce GTX 550 Ti FPB 1024 MB GDDR5 PCI Express 2.0 2DVI/Mini-HDMI SLI Ready Graphics Card, 01G-P3-1556-KR


the motherboard is able to receive a green light when the psu is turned on.

any help is greatly appreciated, i've spent over $700 on this computer and am starting to get worried.