Instructions

To reset the password on your Windows 2012 server, simply complete the following steps:

• Boot from the Micrsoft Windows Server 2012 DVD
• From the Windows Setup menu, click “Next”.
• Select “Repair your computer”
• Under Choose and option, click on “Troubleshoot”.
• Under Advanced options, click “Command Prompt”.
• At the command prompt, run the following commands:
  d:
  cd windows\system32
  ren Utilman.exe Utilman.exe.old
  copy cmd.exe Utilman.exe
• Close the command prompt and then click “Continue”.
• The server should now boot and present the logon screen. Here click Windows Key + U.
• At the prompt you can now change the password, by typing the following command:
  net user administrator Password123
  This will set the password for the Administrator user to be Password123 (case sensitive).

  If a new (adminstrator) user is needed, type the following commands:
  net user “OtherUser” Password123 /add
  net localgroup administrators “OtherUser” /add

Closing the command prompt, you should now be able to log back onto the server using the password you have provided in the last step.

Cleanup Steps

Once you have verified you can log on to the server you will have repeat the steps above and boot using the Windows Server 2008 DVD/ISO and run the command prompt again.

• Restart your server and once again, boot from the Micrsoft Windows Server 2012 DVD
• From the Windows Setup menu, click “Next”.
• Select “Repair your computer”
• Under Choose and option, click on “Troubleshoot”.
• Under Advanced options, click “Command Prompt”.
• At the command prompt, run the following commands:
  d:
  cd windows\system32
  ren utilman.exe utilman.exe.new
  copy utilman.exe.old utilman.exe
• Close the command prompt and then click “Continue”.

You should be back up and running as if nothing ever happened.

source: http://www.kieranlane.com/2013/09/18/resetting-administrator-password-windows-2012/