Outlook not connected to Exchange server

Assuming the machine can ping the Exchange Server (if not look for a network problem) try using the RPCPING utility (Rpings.exe on the server and rpingc.exe or rpingc16.exe on the client side)in the NT4.0 Resource Kit to check for the RPC conection necessary for Outlook to connect to Exchange Server.

Check your NT accounts too, just to be sure the client's is valid. You can turn on logon monitoring for your Exchange Server's Private Information Store and check the event viewer to see if theclient's logons are denied or don't reach the server.

You should check if you have the correct host IP address and subnet mask on your network protocol TCP/IP.
Check this first and then your Lan connections. How are they set?
Also, as a point of information, what kind of system do you have.

first make certain that the user name and password is the correct account for the mailbox in exchange.
2nd verify the the outlook setting is not set by default to use outlook off line or a boguz account setting is set for the profile on the workstation.