connect onto the router with a cable to your computer/laptop and go to the routers IP address (usually 192.168.0.1 or 192.168.1.1 etc) and you should get a login page. The login details may be on a sticker on the router, if not may need to try the default password for the make of router. Once in, there will be a page/tab for the wireless key and you should be able to display it or change it.
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 ***** Username and Password may also be located on the side or bottom of your router *****
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.