User Accounts issue

Between my Son and Ex, they seem to have somehow deleted the User Sign-on linkages.
Everything goes to Default with no Personal Account data.
First error message to the efffect of "User Data not found".
Is the User Info recoverable at this point?
Her Virus is Bit-Defender, but it does not seem to be enabled.

Let's go slow here - when you boot the computer, what EXACTLY do you see?

I'm not there, but will try and get some screen shots if mail is still available.
Thank you for the response!
jeff