drop to a good old shell prompt
type ifconfig
read your ip address for youe ext interface if it scrolls of the top of the screen do
ifconfig ethX
where X=your external interface
Log into your account at
www.dyndns.org go to your virtual domains and change the ipaddress shown to the one gleemed from ifconfig or the one on your primary domain @ dyndns (ifconfig and primary will be the same if not your primary email wont work)
Let us know if it fixes it