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Check to see whether Windows is still running when the PC shuts down. If you have any LED lights on your keyboard and mouse, they are of great help. If they stay on, that means Windows is still running and it's a GPU problem, but it could be corrupted driver for the GPU. Simply uninstall the driver and reinstall.
If the PC completely restarts, get a power supply tester to check whether the power supply is faulty.
Also, try Driver Verifier. It's not a 3rd party software, it's a Windows feature. Driver conflicts can cause the PC to restart itself.
Your post isn't clear enough for me to diagnose the problem. If the PC gets stuck before it loads Windows up with an underscore, it might be looking for a boot device. Like what others said, go in BIOS and check out the boot devices.