What type of router is it, you can reset passwords on Cisco's / 3com but it is a long process.
Otherwise take the router out of service and use a multi-ping tool to ping an interface of the router until you find it's IP, or use a network sniffer to examine the packets its transmitting to get it's IP address. But if you dont know the password anyway then the router may have to be wiped of its config and reconfigured.