You may be able to get the key. You will need to log onto the router using a wired connection - that is, connect an ethernet cable from your computer to one of the ports on the router. Then log into the router by opening internet explorer and entering http://192.168.1.1 on the address line (assuming you didn't change the router's IP address. ) When you get the login screen, leave the username blank and enter "admin" for the password (again assuming you didn't change the password).
Now you have access to the setup program and should be able to see the key if it is displayed. However, it is possible that the key is displayed as *, in which case you will need to provide a new key.
Go to the Wireless menu then go to security and create a new password.