The Problem: You forgot the password to manage your router. Period.
Quick Fix: You have to reset the router back to its factory default settings, and ou'll lose all your configuration settings. On the back of most routers, there's a recessed Reset button. Using a paper clip, hold this button down until the LEDs blink. Once you've reset the router back to factory settings, you can use the default username and password again.
Many current routers allow you to save the configuration settings so you don't have to reconfigure after performing a factory reset. Check to see if your router has that capability. If it does, save the settings now.