You could have an error in your zone files (/var/named/*.db) or in your main configuration (/etc/named.conf) or you could possibly have access to DNS blocked or it could be something as simple as the DNS service (named) has not been started and is offline (common after reboots):
What does the following tell you?
Code:
# service named status
# service named restart
Also, anything useful from these?
Code:
# tail -50 /var/log/named.log
# grep "named" /var/log/messages | tail -50
If you got CSF or some other firewall installed, make sure that you have not blocked port number 53 as that will kill your DNS.
Similarly, if you harden your /etc/named.conf configuration and restrict the recursion and transfers, make sure your resolvers are whitelisted else you will again cutoff access to your DNS servers.