WiFi key or the admin password?
If it's the WiFi key - connect with an Ethernet cable. Open a browser ago go to
http://mywifiext.net where you can change the settings (after logging in with the admin password).
If it's the admin password - factory reset button is your only option. Get a paper clip or something and hold it down for 10-15 seconds.
Default admin login is for most Netgear equipment is:
Username: admin
Password: password