Computer shuts off randomly and fails to restart itself

Hello,

I was on my pc a few days ago and it shut off instantly, like the power cable had been taken out. After this it instantly restarted its self and worked fine. It then did it later on;however, after it restarted its self it shut its self off. This repeaded its self a few time before I stopped it. Now a few days later I can't find a problem with it, it just keeps happening. I tried replacing the hard drive ( my brothers ) just to see if it worked and it sometimes loaded but still shuts off. Now very often when it starts and doesn't shut off immediately my monitor had a blank screen.

pc specks:

i5 2500k
PSU: Corsair Rm 750
Gpu: 660 ti
Ram: ddr3 vengeance 16gb
Corsair water cooling h100i
Mother board: Z77 extreme 4