i have same prob last night but now solve

1 ) restart server

if prob not solve

2 ) restart DNS Server
Code: 
/etc/rc.d/init.d/named restart
if prob not solve

3 ) edit this file
Code: 
nano /etc/resolv.conf
and add your server ip

Code: 
nameserver ***.***.***.***
nameserver ***.***.***.***
chang * to your ip

4 ) Reboot Server