Computer shuts off randomly

[Kravs]

My computer shuts off randomly when I'm not on it, and when I try to turn it back on, it powers on, then powers off, then powers on again, and loads my Bios.

There was a time where it shut off and it literally did not turn on for a whole week. I figured it was my power supply but then after a week my computer turned on. 

 

GPU: GeForce GTX 750 Ti

CPU: i5 3570k @ 3.40 ghz

ASRock z77 extreme 4 motherboard

8 gb ram

corsair cx 500 power supply

 

I'm very stressed out and don't know what's going on. My computer starts up extremely slow. I used to be able to run GTA 5 on high graphics and now I have to run it on EVERYTHING low.

I do not have a clue as to what the problem is and I would appreciate some help. Thank you.

[MuHammer]

check the windows eventlog first of all look at the time it shutdown