Windows 2000 workgroup to domain

I had a winnt 4.0 server that crashed so I loaded win2000 server on it giving the same computer name and domain name as the old winnt server. I also have another win2000 server on the network that was just a basic file server that was part of the original winnt domain. I took it off the domain and made a workgroup so that some of the computers in the network could access files from it while I worked on the crashed server. When I put the crashed server (with win2000 server on it)back on the network I can get all of the computers to join the domain except the ones that I set up in a workgroup. They can access it through my network places or by mapping a drive just not join it. Any ideas? Any help would be appreciated. Thanks!