In the middle of a Windows 10 update, the computer restarted itself but wasn't able to boot back into Windows with a boot device not found error. I booted the machine to a Windows Media Creation Tool to attempt to repair the update without success because none of the options available to me (repair, reinstall, rollback) were even permitted. I removed the HDD from the computer and plugged it into a working Windows machine (using a SATA-USB adapter) but could not even discover the drive in Windows Explorer, despite being recognized by Device Manager.
UPDATE
I originally thought, for a number of reasons, that the update was responsible for modifying the drive's boot sector but one of the contributors was actually correct that the Windows update merely exposed a faulty HDD and consequently put the drive in an unbootable state. This was in part confirmed by the Windows disk management tool which exposed a cyclic redundancy check error.
If this has happened to you, the drive obviously must be replaced and a specialized data-recovery program is likely your only hope at data recovery. I was able to recover everything using Stellar Data Recovery which I now highly recommend.