Could be 1 of one million things like -
- Power supply is insufficient to run all devices. Remove external devices like usb drive, phone, external HDD ... still nothing? disconnect cd/dvd device....Still nothing?...
Have you upgraded Hardware recently?
- a new video card?
- more Ram?
Did the computer shutdown unexpectedly due to a blackout?
- your bios settings may have been reset to default and Windows doesn't like the default settings.
Are you Gaming online?
Are you gaming at all when the shut down occurs?
Are you watching YouTube Videos online?
Are you doing anything with Videos eg: watching, editing, compiling, decrypting?