If this is a SOHO NAT router (D-Link, Belkin, etc.) and you have not disabled your DHCP server, then re-acquire the DHCP configuration by either rebooting or releasing and renewing. You should then be able to access the NAT router's web management interface by heading over to your default gateway address via the web browser.
Set manually your ip address on your computer, like 192.168.3.20, mask 255.255.255.0.
You should be able to reach 192.168.3.1.
If this is a SOHO NAT router (D-Link, Belkin, etc.) and you have not disabled your DHCP server, then re-acquire the DHCP configuration by either rebooting or releasing and renewing. You should then be able to access the NAT router's web management interface by heading over to your default gateway address via the web browser.