When restoring a Domain controller, it's recommended to boot into Active Directory Restore Mode on first boot. This will require the initial password when the server was first built. It's recommend that if you're unaware of what this is, to reset it by following the steps below.
When a Windows Server 2012/2008/2003/2000 machine is prompted to a domain controller (DC), the Directory Services Restore Mode (DSRM) password is created for the local administrator account. This password will be used only when booting into the recovery console or Directory Services Restore Mode. If you forget the DSRM password, you can't use the recovery console nor restore the Active Directory (AD) database.
Reset DSRM Password with Ntdsutil.
You can now use the local administrator account to log on to the recovery console or Directory Services Restore Mode using the new password. However, you can only use the Ntdsutil utility to reset DSRM password while logging into domain controller. If you also forgot domain administrator password, then you need to use the method below to change your forgotten DSRM password.
Please also refer to our support document "Special Steps required when restoring an Active Directory Domain Controller"