Is there anything in the Eventlog?
If it says unexpected restart, this normally indicates hardware or external (eg power dip).
Please confirm you didnt do any BIOS updates recently.
Some Seasonic PSU’s are known to have an instant power delivery issue. I have one too (750W), and have had 2 ‘insta’ reboots when running MSFS, but nothing since. This was just after getting my 3080. I doubt a 1080 will do that though.