if you press the button on the back (if it has one) it should print out a config page where you can find out what ip it's set to. If not, the standard ip addresses of them are 192.0.0.192 so you'll need to put a pc on the same address range segment as it to be able to communicate with it so you can reconfigure it... 192.0.0.193 for instance. then type in internet explorer
http://192.0.0.192 to access the web config of the JD card
failing that, download HP Web Jetadmin software from HPs website (free download) and that will detect and set it up for you (just a bit of a pig to install)
If it's set to get an address from DHCP check the status page of your router, it may be showing up in there as occupying an address lease. if so, browse to it in IE same as you would on one of the earlier approaches (
http://x.x.x.x)
Brilliant printer by the way, they will out live us all
Laserjet 4000/4050's are the same. Pity the same cant be said about some of the current models