What could cause this problem? (screen turn black then pc restart)

korialz

Honorable
Jul 28, 2015
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So what could cause this problem? This only happened right now.Never encountered this before..so everything in my system is brand new only the hdd is the old one. So this happened while I tried to play League of legends...I just entered the rift/the game/map then suddenly my screen turns black, could still hear the sounds for a minute then it turns quiet.then pc restarts. as the pc boots up I then quickly opened lol again to go back into the game but same thing happened..as soon as I entered the rift/map the screen then turns black and I know that it will restart I just pressed the restart button manually.
this time im thinking that my graphics card has a problem cuz as I know if you are playing a game(as the assets are loaded) and suddenly your pc shuts down or having a problem then it might be a graphics card problem.so this time I just opened Witcher 3 to know if its the graphics card but then Witcher 3 runs perfectly..I then tried again lol and this time I could now play the game.so what gives? what happened there ? what caused the black screen/restart? by the way I sometimes have 100% disk usage even though im not using anything..so due to this I am currently checking my disk for errors and defragmenting it..so is this due to the hd ? and not the graphics card or is there other thing that might cause this? im just making sure that my graphics card is still ok....

update: forgot my spec:
Asrock Z97X Killer Motherboard
Intel Core i5 4590 Processor
Seasonic G-550 80Plus Gold
zotac GTX 970
24 gb ram (2 4gb(old) , 2 8gb(new) .all fury)

by the way..I just recently added the 2x 8gb ram.....
 
new pc build...make sure the mb bios is up to date to rule out a bios bug. in the bios make sure ram set tot xmp profile. use msi afterburner make sure the gpu temp is fine. use hardware info 64 bit set it to sensor and logging make sure the cpu temp is fine and power supply 12v rail is holding above 11.7v. look in the bios make sure the 12v rail stays above or at 12v. if you have a cx line of power supply there known to not hold up under load.