This usually happens when the system is setup to automatically login to an account, and the account is deleted.
To resolve this...
Win-X -> Command Prompt (Admin) -> control userpasswords2
This will let you enable or disable automatic login.
Check the "Users must enter a username..." checkbox, hit "Apply". This will disable auto login.
If you do want your tablet to login when powered on then uncheck the box, and hit "Apply".
It will ask you for a username and password to automatically login the system when powered on.