Missing domain

[netboss at cyberport.net]

I need some help determining a problem we are having with a domain, z600.com.

nslookup done on our name servers produces the correct results for this
domain.  nslookup using any other server produces the error "Non-existent
host/domain" just as though the domain were not present in the root name
servers.

The problem is that the domain has been registered and working until a few
weeks ago.  We have checked the entries in Network Solutions for the
domain, and all appears to be correct.  The domain is not up for renewal,
and the DNS entries are as they have been for several months.

How can I determine that a domain exists in the root name servers?  If the
domain is not in the root name servers, but everything indicates that it
should be, what is the next step?

Thanks for any help offered.

Gary
CyberPort Montana