Usually, this is either caused by an insufficient power supply or by insufficient cooling of the CPU. I'd check the heat paste and all fans.
I'm not sure why but occasionally my PC will just stop. No blue screen or any notice, it just turns of. After a sew seconds you can switch it on again. It seems to primarily occur when I start a game (battlefront 3 or fallout for example).
CPU: AMD FX8350 Black Edition 8 Core Processor
GPU: Sapphire Radeon R9 270X 4GB 1020MHz GDDR 5 PCI Express Graphics Card
CPU Fan: Zalman CNPS10X CPU Cooler
RAM: AVEXIR CORE BLUE SERIES 16GB (2X8GB) DDR4
Memory: 240GB SSD. 1.5TB HDD
Windows 10 (64bit)
My apologies if I've forgotten anything