Computer crashing while playing games/watching streams on Twitch.tv

Hello there,

This all started when I was in class and let my friend on my computer. I don't know what he did but after that day, I have been experiencing crashes. For example, I play League of Legends, and it run fine for a few hours. The first crash I remember was playing LoL for 7 hours and bam, suddenly a black screen appeared. I was on skype with friends at the moment so I knew my computer crashed and it was not just a black screen. It then auto rebooted and was fine for another few hours... More crashes occured and I remember getting a message saying Windows has recover from an unexpected shutdown. It said something about blue screen but I never saw any. I thought there was an issue with overheating so I put a fan next to it with the case open, but still crashes. While typing this, I just experienced another crash. I was playing League Of Legends as well and had this window in the background. This time, it was just a black screen. I looked inside my case and only saw the case fan and psu fan running. I checked the motherboard and the power supply wire connected to the motherboard was loose and fixed it. I don't know much about computer and this is my first computer I built. It was working fine for about a month never experience crashes like this.
 
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