Are you using a dynamic or static LAN IP address? I ask because if you are using a dynamic LAN IP it is possible that the DHCP server on your router has assigned you a different IP address, invalidating any port forwarding or DMZ settings. Only a static LAN IP configuration should be used for reliable port forwarding or DMZ use.
Are you using port forwarding or the DMZ? (using both will not work and should not be done)
If you are unsure of how to configure your LAN IP address statically outside the DHCP range of your router then you can check here:
http://www.portforward.com/networking/staticip.htmfor some basic info, but it does not specify the difference between a static LAN or WAN IP.
Before following those instructions read your router manual and adjust the procedure accordingly.
What model router are you using? Have you read the manual for it? (particularly the static LAN configuration and port forwarding sections)
Also what is the result of an "nslookup client.sfc.dynaverse.net" at the command prompt on your machine?
(I have half a mind to produce a router with firmware and client software that walks the user through static lan configuration and portforwarding... there is a clear need for it, as the average user rarely reads the full manual for their router (if it is even provided) and does not understand or care to research network configuration (nor should they necessarily be expected to) I'm sure there's a buck to be made here somehow... hmmm...)