If you are restoring a Domain Controller, you will need to perform additional steps when booting for the first time after recovery. Simply restoring a Domain Controller to an earlier point in time is insufficient, as the directory is replicated to multiple Domain Controllers and time-stamped with a serial number to identify the latest version.
For example, on the first boot after recovering the Domain Controller, you will need to press the <F8> key and then choose Directory Services Restore Mode from the boot menu. You will also need the original Directory Services Restore Mode Administrator password created when this server was promoted to a Domain Controller. If the Customer does not have this password, contact Microsoft for support.
For complete instructions, please reference the Knowledgebase.