Problem with Exchange Server 5.5

I have a Microsoft Back Office Server v4.5 with Exchange Server v5.5 installed.
For months there weren’t any problems.
Just a few days ago, users couldn’t access the mail server.
Outlook 2000 reported a time out on the server.
Also some mailboxes on the server where changed (the mailbox name and the site name)
If i install outlook 2000 on the server itself, i can reach most of the mailboxes, but from workstations in the network it is impossible.
People can browse to the server, log on to it, and access shares.
It is a major problem at the compagny.
I need some help with this….