From the command prompt, type this, notice the space: ipconfig /all
Look in the results for the IP address for the "Default Gateway", that is
probably your router's address.
Take that address and put it into your Internet browser. It will possibly
prompt you with another box asking for your username/password. This is not
the same as the one you were looking for before.
Usually, the username is Admin
The password varies, and of course if you changed it there is no one that
knows it here. You may have changed it during initial setup of the router,
perhaps when running a CD that came with the router.
If you do happen to get into the router setup, look for "Wireless" or
"Wireless security" for the passphrase/key.
Good luck.