My computer keeps freezing

So yesterday, my computer turned off for no reason. I thought it was overheating, because I had the computer had been on for 10 hours straight (My brother used it as well). Today, I tried turning it on, and it freezes at the start, where, for my computer, it says GATEWAY, and big words. It stays there and I can't do anything about it. After 10 minutes, it beeps once, and a minute later turns off. Also, yesterday the screen was black when I left and came back for a while, and it wouldn't turn back. I pressed enter and it made the login sound, and then I can hear some sounds from the game my brother was playing, but it remained black. So I turned on High Contrast and turned it off and it turned back to normal. Does that have anything to do with it? Help?

Oh and by the way, I can't access Safe Mode. I don't get a BSOD either.
 
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