dcpromo /forceremoval

If you have removed a DC from the domain, dcpromo w/out the /forceremoval switch will fail since it cannot contact the domain to notify of its demotion.
Don't forget after you do a /forceremoval you have to clean up your Active directory.
Also if that DC held any roles you will have to seize them to another DC.

332199 Using the DCPROMO /FORCEREMOVAL Command to Force the Demotion of Activehttp://support.microsoft.com/?id=332199

251307 HOW TO: Remove Orphaned Domains from Active Directory Without Demotinghttp://support.microsoft.com/?id=251307

216498 How To Remove Data in Active Directory After an Unsuccessful Domainhttp://support.microsoft.com/?id=216498

Also see http://www.petri.co.il/delete_failed_dcs_from_ad.htm

255504 Using Ntdsutil.exe to seize or transfer FSMO roles to a domain controllerhttp://support.microsoft.com/?id=255504