Over and over again- the dreaded boot loop.
Here's how to fix it*. (The bad news is you'll need access to another Windows 10 computer, assuming you haven't already created a Windows 10 installation DVD or USB drive.)
- Set the computer to boot from USB or DVD, depending on the media you created, either from the BIOS, or by tapping F8 while booting, which on this computer gave me boot medium options.
- Boot from the Windows Media DVD or USB drive you created.
- Click on Repair your computer at the bottom left of the window.
- Click on Troubleshoot > Advanced Options > Command prompt.
- Type in "diskpart" and wait a few seconds.
- In Diskpart, enter "list vol".
- Look for the volume label of the main partition on the HD- it will be NTFS and many GB. On this computer it was E.
- Type "exit" to get out of Diskpart. Run Chkdsk on the main partition. In this case it was "chkdsk e: /f"
* The root cause may be an interrupted power supply or a failing hard drive. In this case I suspect a faulty power cable. If disk errors are the result of a failing hard drive, the fix may be temporary.