New pc won't turn on

[www.mastergemmer]

Please help me!I just assembled a brand new pc.When I turn it on,the led fan turns on for a fraction of a second and then goes off.I can also see dim lights in my mouse.It does not turn on at all.

 

[piechockidocent9]

Don't panic, just few days ago my rig did exactly same while all I did was one fan replace. Double check every connection to your psu, make sure everything is firmly plugged all the way (on both ends if you have a modular psu) and you did not forget to plug in something. Won't hurt to take a look at connectors while doing it, dust/blow it off a little in case anything got in there.
If that won't do I would advise trying exchanging psu->mobo power cables with modular psu or try another psu.