A Simple Problem of LAN & INTERNET

Could anyone help me.
I have a simple LAN setup in my office with Win 2K Server & XP Terminals with BUS topology. The problem occurs suddenly. All XP terminals can not access Server and other terminals as well even when Drives are shared. Even Server machine can not browse other machine. But all machine can exchange Emails through our Server through MDaemon. All XP Terminals can access internet. XP Terminals are connected through a Hub. Can Anyone help me. Thanks.