locked out of W2K

On a networked laptop with W2K professional, changed user from a domain to a newly created workgroup. In attempting to return user to domain, it asked for user name and password. His user name and password could not be authenticated. This laptop is part of a large intranet that connects by VPN through DSL. We were trying to create a small in office intranet using a WIN NT 4 as a print server connecting the 3 employees thru a hub to the server to share their printer.
The laptop was rebooted and asked for user name and password without the domain showing up or even given as an option to choose. The user’s name and password will not work. I am at a loss as to how to get his PC back to it’s orginial configuration. Please HELP!