Attached is some more detail about the solution that worked for me. Posters in another thread asked for more details about my post.
As described in the initial post of the thread, when I tried to boot the computer, an Automatic Repair screen runs, then a Diagnosing PC screen. The Repair/Diagnosis fails and I end up on a blue screen that says the problem couldn't be diagnosed and provides options to "Restart Computer" (which just leads into the same loop) or "Advanced Options." I hit "Advanced Options", then "Troubleshoot", then another "Advanced Options", then "Startup Settings" (on this screen there are also options for "Reset System", "Restore Previous Build" and others, all of which failed), then "Restart", then (finally) "Disable Early Launch Anti-Malware Protection". Once I hit this, the computer started fine, and I was able to view all my files and apps and use the computer. However, when I shut down, the problem occurred again, so I had to run through all the same selections again. After that, I uninstalled my virus/security software (Norton 360) and the computer came on without issue, so it seems like the virus/security software may be the issue for me. I have not yet tried to reinstall the virus/security software.