If the machine has been disconnected from the network longer than your Active Directory (AD) forest's tombstone lifetime be sure not to allow the machine to actually replicate with any existing domain controllers (DCs). (You can get the tombstone lifetime by running dsquery * "cn=directory service,cn=windows nt,cn=services,cn=configuration,dc=<forestDN>" –scope base –attr tombstonelifetime
, substituting in the DN of your AD forest, obviously).
If you don't have a working password from the AD instance running on the machine boot it in Directory Services Restore Mode (DSRM) and logon with the password set when the machine was promoted to a DC.
If you don't have the DSRM password you can reset it with any number of third-party utilities (I like http://pogostick.net/~pnh/ntpasswd/, personally).
(There are any number of other "hacks" that will get you onto the machine, but that's how I'd do it, personally.)