1)Open the web browser that you use: Firefox,Google Chrome,IE,or Opera and type in the IP address for your router,and press the Enter key. http://www.routeripaddress.com/< Router ip addresses can be found here
2)A window will open and then type in your routers default username and password,and press the Enter key again http://www.routerpasswords.com/< Router usernames and passwords found here
3)Navigate to the Wireless Security\Network Security Settings Tab and open it to locate or change your WEP or WPA Security Passphrase. Use WPA because it provides better security.
4)Write this Security Passphrase down and put it in a safe place to access when needed.
I just dealt with this issue. When asked for a security key initially I used the wrong key/password and received an error message from my provider with the Header - Frontier 2712. The 2712 is an error code. In this instance and because there was an error code - this told me I needed to enter the WPA # on the back of my Router. Mine was actually more letters than numbers but that got me reconnected immediately. Hope this helps.