DNS server could not resolve, no response from root servers

[Yihua Philip Sheng]

I found a very strange thing. My DNS servers (both master and slave) 
suddenly could not resolve most of the domain names.  I am running BIND 8 
coming with the Solaris 9 OS. But

Runing the "snoop" command found that the root servers are not responding to 
my DNS server. I only saw my servers sending requests out to the root 
servers, but seldom saw root servers respond. My network guy told me  that 
the port 53 is open on his firewall.

any idea? Thanks a lot,

Philip