Before upgrading your Windows 8 PC to Windows 10, you need to disable "Secure Boot" in the system's BIOS. This "feature" is designed to prevent booting from CD's DVD's Flash Drive's, etc that may contain viruses. It can also cause your Windows 8 PC to boot to a black screen in the middle of the Windows 10 Upgrade process. If this happens, you need to edit your system BIOS, disable the Secure Boot and enable Legacy Boot. The exact steps for doing this are dependent upon your particular model of computer. Once you do this, you can restart your computer and the Windows 10 upgrade will pick back up where it left off.