If the machine is still a member of a domain (i.e. if you did not remove the last and only DC from the domain), you should be able to log in with DOMAIN\administrator and the domainadmin credentials.
If it was the last DC of a domain or you can't use the domain admin account for some other reason, you need to use the "MACHINE\Administrator" user and the password you've specified for "Directory Service Recovery Mode" during promotion.
If this password is lost, reset it by using a
ntpasswd boot disk or live CD and try again.