Add User/Computer to Active Directory

I got a job working in a small office setting maintaining their computers. The previous tech set the office up as a domain, no workgroups. On the Windows 2000 server I set up a new user/computer but when trying to connect the new user/computer workstation to the server it won’t go. I put in the user name and domain name in the network setup but get the error message “can’t find the domain controller.” Any ideas? I have never worked with a domain before, just workgroup setup. Thanks!