To reset the domain admin password you will need physical access to the server or access to the virtual environment. In this example I will cover Windows 2012 procedure. Place installation DVD in your server or mount ISO file using your virtual machine settings.
1. Restart your server, and make sure to boot from the installation DVD.
2. Once setup is started, select language, keyboard layout and time settings and click Next.
3. Now select option Repair your computer.
4. Once in Repair section, select option Troubleshoot
5. Select Command Prompt.
6. Once you get command prompt window change back to D:\ drive and navigate to Windows\System32.
Now we will replace Utilman.exe with cmd.exe. I cannot stress this enough, please make sure to make a backup copy of the file you are replacing!!! So,
copy Utilman.exe Utilman.exe.orig move cmd.exe Utilman.exe
Once prompted, confirm overwrite
7. Restart your server using command shutdown -r -t 0.
8. Boot into Windows
9. When login page is loaded, press Windows key (Windows logo key) and U. This will bring Command Prompt windows up. Now, use the following command to reset domain admin password:
net user Administrator "YourNewPassword"
Obviously, replace word YourNewPassword with your actual password. Since this is done on the domain controller, and you have just changed password of the user Administrator, password for the domain admin is automatically changed. Now you need to roll back change you have made to Utilman.exe file, so, repeat the steps 1 to 6 with exception to use ONLY command move Utilman.exe.orig cmd.exe and confirm when prompted.