Probably a Windows update. Allow Windows to perform its regular update before attempting to troubleshoot.
Rebooting could be due to Hardware failure. Hardware failure or system instability can cause the computer to reboot automatically.
If the issue is hardware - The problem could be the RAM, Hard Drive, Power Supply, Graphic Card or External devices: – or it could be an overheating or BIOS issue.
A lot of troubleshooting these kind of problems is trial and error with no clear guaranteed steps. If you're not sure how to identify the problem yourself, get in touch, we can help! Create a ticket for a phone support call or site visit by submitting your information here https://demidov.freshdesk.com/a/tickets/new