Mine won't help you but yours is visible in the router's settings pages or on some routers, displayed on the outside of the case.
If it isn't that obvious, press your Windows key and R together and into the Run box, type
cmd /k ipconfig /all
then press Enter. The black form will show your Default Gateway and that's the address you need to type into the address bar of your Net browser - Internet Explorer, for example.
Once you're there, you need a login and a password - usually admin for the login and password for the password. Get past that stage and under Wireless and Security, you should see the wi-fi password - known as the security key - should show up.