Why does my PC restart itself a few seconds after start playing video games?

After opening a video games, after a few seconds of play time, my PC restart by itself. And it’s not just one game, but a few games I own did that even though it was okay this morning.

Other than playing video games, my computer sometimes still restart by itself but in a few hours.

I’ve been trying to checkdisk, defrag, memory diagnostic but I still has the same problem.

Please, if anyone know the solution, tell me please so I can fix it.