Use the following steps to active administrator account.

1. Open the command prompt as Administrator

Normally, the command prompt can be opened as a regular user to run commands that don’t require administrative rights. However, if you need to run a command that requires administrative rights, you must open the command prompt window as administrator.

• The first method of accessing the command prompt as administrator is to right-click on the Start button    in the lower-left corner of the screen and select the Command Prompt (Admin) option from the  User menu.

You can also press the Windows key + X to access this menu.

–  The second method involves the Start screen. If you are currently on the Desktop, click the Start button in the lower-left corner of the screen.

On the Start screen, start type “command prompt” (without the quotes). The Search panel displays on the right side of the screen and results of the search display as you type. Right-click on Command Prompt and select Run as administrator from the popup menu.

So now you are opened the command prompt in administrator mode.

2. Type below given command to see the users list:

C:\WINDOWS\System32>net user

3. Unlock the administrator account.

C:\WINDOWS\System32>net user administrator /active: yes

4. Give a password.

C:\WINDOWS\System32>net user administrator *

(* – type the password)

Source: howtogeek