The network key is set on your wireless router/ wireless access point, so there are two ways you can go about this.
1.Access your wireless routers setup, they usually have a web interface that you access by typing an address in your browsers address bar, usually something like 192.168.1.1 (see your manual for address.) and then you can go into the wireless setup and see the key that you set
2. You can do a hard reset of your router to reset it to factory standard settings and then set up your wireless connection from scratch.
Hope this helps