Have you tried restarting the DNS server via Server .. Services .. Service management? Maybe it's stopped (in which case it will show up in that screen).
Secondly, if you tried that and it still does not work, check out your domains and name server via dnsstuff.com. That should give you a pretty good idea of the reason of your problem. If you had mentioned a domain name in your message, I would already have checked it myself...
Ron