I've just had to diagnose a family members laptop which they could no longer log into, their Username and password had been changed without their knowing.

What I believe happened is that, originally I had set up their laptop with a local user account (yes this is possible, but microsoft make it really hard) , and they recently signed into a Microsoft app, which then converted their local account into the Microsoft one. Which in turn changed their local Username and password to match their Microsoft account. Probably without even consent but just as likely they pressed agree to something they didn't understand.

I was unable to undo the damage and restore it to the previous state but it was not easy.

Take care fellas, love kara.