After upgraded Windows 8 to Windows 10 today, Windows 10 automatically disabled the built-in administrator account. I think this was Windows 10’s new security policy. I could activate the administrator account from the other administrator user account. If you are planning to upgrade your old Windows machines such as Windows 7 or 8 to Windows 10, you should create a user account with administrator authentication before the upgrade. You may have a trouble to login after the upgrade. Here is what I did.
Login to the oather Windows 10’s administrator account and type “cmd” in search box. Click to open the command prompt “Run as Administrator.”
net user <your built-in administrator account username> /active:yesPress enter and reboot Windows 10. That’s it. You will see the built-in administrator’s username on login screen after the reboot.
If you do not have the other administrator account, you hold “Shift” key and reboot PC. Then, Windows starts recovery environment. Choose an option screen –> Troubleshoot –> Advanced –> Command Prompt. When you get the command prompt, enter the command above and then reboot. You should be able to login as the built-in administrator at the next login.