**** little info about your equipment.
If you just replaced the router with an Airport Extreme or Express:
disconnect the modem from power, disconnect the new router from power, connect the modem to power and wait till it os fully started up, then connect the new router to power. After a while the amber light will turn to flashing, then after a while it will turn to green. Then with AirportUtility (in Aplications/Utilities) it should be configured with the password.