Started Defrag and let it run for about 90 minutes. I chose to stop the process so I could turn off the computer. I pressed the cancel button and waited for the application to fully stop, then turned off the computer. The computer doesn’t start now. It begins to boot, gets past the Windows icon, then an error appears: “Sign-in process initialization failure: Interactive sign-in process initialization has failed.” The computer runs Windows 10. I haven’t been able to use the computer since about August, 2024.
If you did as you say and cancelled the defrag, and waited for it to fully stop, then that should have been fine.
(However if a drive is faulty anyway then the extra read/writes of a defrag could push something over the edge).
The error message that you are seeing is often/usually shown when you had your Windows account sign-in set to ‘no-password’, ie,automatic sign-in, and that automatic sign-in process has somehow become corrupted.
(Whether that was caused by the cancelled defrag, or was just a coincidence I couldn’t say).
It may be just a missing/corrupted dll file (possibly defrag tried to write it to a bad sector on a faulty drive?), or it may be a bit more complicated.
There are a few things you can do to try and fix it, starting with a simple Alt+Tab+Enter which can sometimes get you past that error screen.
However at the other end of the scale you may end up having to log in as the ‘Hidden’ Windows administrator to copy/replace the account that now can’t sign-in automatically.
See the pretty comprehensive reply here which has links to other articles/fixes:
https://answers.microsoft.com/en-us/windows/forum/all/sign-in-process-initialization-failed/5ed2784d-a8a9-4f41-b0d4-879b359d1ea9