In the left pane of Event Viewer, double click/tap on Windows Logs to expand it, click on System to select it, then right click on System, and click/tap on Filter Current Log.
Check possibly for a 6008, 6006, 1074, or 41 event. Event ID 6008 is for a forced shutdown. Something is forcing your computer to shutdown. While 6006 is a clean shutdown.
for 41, See: Advanced troubleshooting for Event ID 41: "The system has rebooted without cleanly shutting down first"
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Regards,
Abiola