Kaylangan mo ng installer kung wala ka download ka ng installer ng w10 then saksak mo yung usb tapos meron atang pang factory reset dun kung yung files mo kunyare steam games mo nasa ibang hdd alam ko di ma wawala yun pero lag isa lang hdd mo ehh mawawala yun. For the most part yun lang ang alam ko kung paano ayusin to
Para ma-rule out mo yung issue kung hardware vs software (OS), run a Linux LiveCD and use it for a bit and see if the machine reboots as well. If it does, most likely it is a hardware issue, eg PSU, RAM, CPU, overheating etc.