I checked the authoritative DNS server's resolv.conf, it has the IP of the DNS server and the ISP's IP as a secondary server for back up. I remove the ISP's IP now the DNS server cannot access the internet. But it doesn't make sense as the website that it is hosting right now, is working fine.

I tried the dig, host and nslookup. The only problem is the nslookup for the domain that uses our nameserver. It gives this result:

~# nslookup www.somebodysdomainname.com
; Got recursion not available from xx.xxxx.xx (ip of the DNS authoritative server), trying next server
; Got recursion not available from xx.xxxx.xx (ip of the DNS authoritative server), trying next server
Server: IP of the slave DNS server (my slave server)
Address: ip #53

** server can't find www.somebodysdomainname.com: SERVFAIL

Well, I don't know how to fix this. I check the resolv.conf, it has the IPs of my DNS servers and now I cannot connect the internet. But the website I am hosting can be accessed fine.

Can you further help me with this please?

Thank you so much.