My iMac suddenly restarts without a reason

Hello, I have a pretty old iMac but it worked fine for about 1.5 years since someone gave it to me. Now when I open it after some time not even 10 minutes its just restarts on its own. No error message no nothing. I think it does it only with games. I've tried running youtube on it for a couple of minutes and it was ok but when I opened Minecraft loaded and into a world it just closed. I don't know whats causing it I don't know even if its the games fault or something but if someone could help or had a similar problem and know how to fix it I would appreciate it!