My Systems Are Not Browsing Please Help!

I beg Network gurus in the house rescue me. i have a network of 50 systems with a server on win server 2003 platform, just suddenly this morning the systems stoped browsing exception of the server. all my troubleshooting skills have been exhausted trying to find de solution. please help me, i checked whether the antivirus was blocking access to the net but found nothing. there is the local area connetion but not internet connection.
please help. one morething, the ip address generation is dhcp .
infact, its snot funny here.
thnx

Furthermore, when i ping the clients from the server, there is reply, but when pinging the server from any of the client, there is no response
its serious o.
please help