Weird noise

Hey.. Whenever I'm playing League my computer makes a weird sound and it just stops working, it won't answer anything, I can only turn it off.. I just got the computer, I don't know what's happening.. Also, I can play 3v3 just fine on twisted treeline but whenever its 5v5 either Summoner's Rift or Howling Abyss it makes that sound! And I don't have skype on or anything, and I tried lowering the graphics.. Does this happen to anyone else?! Is it normal?! How can I fix it?! ... Please help :(