I have discovered that in some cases this problem results from the use of the Windows Feedback app which comes with Windows 10. The app sometimes changes the Windows account from a User account to a dedicated Microsoft account and gets stuck on this setting. I believe this may be because the app requires the user to log in but has no option to log out again.
In my case when I closed the feedback app and rebooted the computer, I was suddenly confronted by a blue log-in screen which had never appeared before. It took me a long time to realize that the password it was looking for was my Microsoft account log in. This was odd considering that I am offline whenever I log into Windows! Also I have never had a password to log into Windows before, not EVER. So I found the sudden appearance of a log in screen very daunting and puzzling.
The solution to the problem is as follows:
1) Click on Windows icon, Settings, Accounts.
2) Click on Change to a user account instead.
3) Enter your Microsoft account password when requested. Click enter.
4) Leave the user account passwords blank when requested Click enter.
Your PC should now reboot to Windows without requesting a password.