Open Internet explorer, firefox or google chrome. It doesn't matter what browser you use. Then depending on your router, type: 192.168.0.1
It shall load a windows asking for a username and password. If you didn't change it it should be admin with no pass, or admin and pass being admin too...
Then you can just run the WWAN wizard to setup wireless again, and copy-paste the passphrase (or write it down on a piece of paper). Click apply then let the router reboot.
Voilà! You only need to enter the passphrase once when you try to connect to your wireless network (if your tick the box "remember the passphrase" of course).
If the IP adress 192.168.0.1 or the user/pass doesn't work, then give us the model of the router and we will help you with it.