We have a problem where we cannot access one of our user areas created on an internal server. We have found that it is an issue with the interrogation of the nameserver as the nameserver itself is working. The virtual dns is working and active and the domains are running but when we try and ping this internal address it comes back as unknown host when we can ping other internal directories without problem.
That sounds like an issue with access control or the zone just not loading. Normally on a public-facing nameserver you would have a set of ACLs defining who can view or use what content.
For example you may have one allowing the localhost to do recursive lookups (lookup anything), another allowing localhost and local hosts to lookup your internal-only DNS records and a final global one just allowing external hosts to lookup domains which you are the authoritative external server for.
My guess would be somehow the definition for that specific zone has the wrong ACLs applied (maybe in the wrong section of your config) or is not loading.
Check it's being loaded ok on the nameserver by looking at the logs and check if you can resolve it from the nameserver itself (usually as I mentioned allowed to resolve anything on the nameserver).
This should point you in the right direction.