Computer shutdown unexpectedly

I left my computer on before I went out because I was running a game, but my friend messaged me that my game got disconnected, and the time I received that message is 7 hours ago, I went out at noon 12pm+ and came back at night 10pm+, means that my game got disconnected at noon 3pm+ (after 3 hours). When I came back, the computer was turned off, not only my game got disconnected. When I start it up, it appears the black and white screen that shows some selection 'Start up normally' like what happens when you start it up after some electric cut off while your computer is running. But I'm not sure if there's any electric cut off when I was outside, this happened for 2 times and never happened before, the last time is around 1 month ago, and it happened again for now, not sure if it's coincidence of electric cut off or there IS something wrong with my computer? Please help. ( Sorry for my bad english :/ )
 
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