Here is the scenario
Mac Mini server in one of my data centres has a fixed IP. Someone has changed the IP on the server and rebooted and now cannot get into the Server at all.
Cannot use local network cable as its a fixed IP that is unknown.
Heres the fix, boot the server into Target Disk Mode and on another Mac edit the Library/Preferences/SystemConfiguration/preferences.plist file.
Look for the ethernet card and there will be an IP address key along with all the other items. Just change that, save and boot the Mac up