There are a couple of different tricks you can do BEFORE trying a reset, which by the way, will not affect fun_plug...
First - download the setup utility from D-Link's website and run it on a PC connected to the same network as the DNS-323 - the utility includes a discovery tool that should, using a network broadcast, locate the DNS-323 regardless of what address has been set.
Second - use a network scanner - for example Fing (from Overlook Soft) - install and run it, again on a PC connected to the same network as the DNS-323 - if the DNS-323 has an ip address within the same network range as the PC, Fing will discover it and display it.
Third - set a static arp entry - look under your DNS-323 and get the MAC address, next, choose an ip address that you know is unused, and on your PC set a static arp entry pairing that ip with the MAC address, and then put that ip address in your browser - don't forget to remove the static arp entry after re-configuring the DNS-323. If you're not familiar with the arp command just open a command prompt, type arp & hit enter - Windows will spit out the help file with examples - look for arp -a