Routers have a special switch that can be used to reset them to factory default, erasing any setting and passwords. It's usualy on the back side, close to the power connector, and looks like a hole with a silver bottom. You'll probably find the word "RESET" next to it.
It's usualy situates in a recess and you need something long and narrow to push it.
Power the router off and wait about 30 seconds.
Depress the reset switch and power up. You may need to keep the switch pressed for up to 20 seconds. (all this is to prevent any accidental reset)
You now have your modem reset at factory default.
You must now reconfigure it completely.
Don't forget to set a new user name and password. This time, be sure to write it down on a piece of paper that you store in a convenient place.
Electro
May 2011