PC restarts/turns off when playing games

everytime i try to load into a fortnite game my pc either turns off as soon as i load into the pre-game island or turns off later on mid-game, same thing happened with GTA 5, as soon as i loaded into an online lobby and walked around for about 4 seconds my pc turned off, sometimes it turns off sometimes it restarts, the only game ive tried so far that works fine is minecraft, i haven't changed anything in my pc and its been working totally fine before this, its also not giving me a BSOD or anything, all help is appreciated :)