freshly built pc - powers on for ~1 second, then resets endlessly

I've put together last week, it's run great until this morning. I push the power button and the fans spin, all case lights and fan Leds turn on, and Hdd spins.
However, after about a second, everything shuts off abruptly and repeats endlessly.

My specs:
Cpu: i5 4670k not oc'd
Mobo: Msi z87-g45 gaming
Ram: g.skill ripjaws 2x4gig
Psu: corsair 750m semi modular
Case: nzxt phantom b/o

Any help diagnosing it would be greatly appreciated. I hate that my first built pc runs great for a week then stops.