I need some help with my pc? please

when i play any online downloaded game like combat arms crossfire etc
after about 15 to 25 minutes my whole computer will cut off with out me hitting the cut off button or shutting it down and then when i do hit the cut on button a black screen will come up and say the system was previously powered down to prevent damage from excessive environ mental temperature i do not know what this means and i do not know how to fix it ive tryed lots of things downloaded anti maleware bytes didnt help any ive tryed multiple anti viruses nothing so someone who knows lots about computers please help me please please i really love playing combat arms but i do not like playing it for 15 to 25 minutes then computer cutting off then having to wait for computer to load bak up then the same thing happening over and over someone please help please please