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I had a problem like this a while back, but it turned out the infrastructure guys changed the server address so I only had to update the server name in Joomla. I would begin with testing the LDAP connection to see what changed. MS has a tool, ldp.exe, or you can look for ldapsearch. It seems to me something changed on the server side and these tools may be able to help you figure it out.
Wish I had something more concrete....