Server Unavailable- HELP!!!!!

I have a user that has not been in the office or logged into his comp in about 3 months. I opened his Outlook (2000) and it said “Microsoft Exchange Server unavailable… The Server containg the Global Address List is no longer available”. I am running Exchange Server 2003 b.t.w. – Where do I begin here? Do I need to uninstall his Outlook on his computer? I can’t even get into it…