If you see a Reboot and Select proper Boot device. Or Insert Boot Media in Selected Boot device and press a key message on Windows 11/10, see this fix.

Look for the following items: Under Windows Boot Manager, the Device item should be set to unknown. Under Windows Boot Loader, the Device and os device items should be set to unknown.

I'm stumped. Yesterday morning my computer was working fine. I stepped away for a few minutes, and when I came back it was on a black screen of death saying "Reboot and select proper Boot device ...