Network cant recognize usernames

We moved a department to another building and network. There were initially no problems when users were first migrated to the new network. This past week when users restart or logoff the network they are unable to log back in. The error message “username cannot be found” displays. Resetting passwords and even deleting and recreating user accounts does not seem to work. I restarted the Windows 2000 server, but no luck. Any suggestions?
Thanks, Mike