Custom built PC keeps turning off...

[Frankenchokey]

Hi everyone, this is my first time building a PC, and also the first time posting on here.
So anyway I built the PC (finishing up last night), it turns on, runs fine, and from what I can tell, all the fans are working correctly.
All Im doing at the moment is letting Skyrim update while doing homework.
I have 1 GTX 770 GPU, an MSI Z87-GD65 Gaming Motherboard, 16 GB of ram (max is 32), a 240 Gb ssd drive, a Rosewill Capstone 750W-M power supply, all installed in a Rosewill Thor V2.
The top of the case had a few extra cables that coudlnt go anywhere (labeled fan a1, fan a2, fan b1, fan b2 etc.) which have been placed behind the leftside case cover. all the fans are plugged into the motherboard and are powered (from what I can tell).
The only thing I can think of is that there was a cable from the power supply that I couldnt find a spot for, it had the plug for the motherboards power (which is plugged in) and plugs labeled CPU1 and CPU2, I couldnt find a spot for both plugs so I just plugged in CPU1.
Also just want to mention that I didnt have a power cable for the 1Tb HDD, so right now its just attached with the sata cable (not sure if this would do anything)
Any advice would be greatly appreciated.

 

[Brantyn Gerik]

the gd 65 has the typical 24pin on the right hand side, but then near the top is an additional 8 pin power connector that NEEDS to be fully plugged in in order for system to boot. also make sure the gpu power cords are properly installed and are plugged into the 12v labeled rail of the psu

 

[Brantyn Gerik]

and YES you absolutely need to have power going to your hard drive, this is likely the source of your whole problem

 

[Brantyn Gerik]

also just since you havent listed it, please make sure a cpu cooling solution is properly installed before trying to run the pc as well (cpu heatsink)