You can always reset the router to factory default settings. Keep in mind you WILL LOSE any changes you have made (whether it be wireless configurations, port forwarding settings, etc). But, if that doesn't matter to you, resetting it will allow you to start from scratch. You must know what your Internet settings should be: whether dynamic IP, static IP, or PPPoE so you can get back online.
However, if your question is "What is the wireless networking (WPA) passphrase?" you can change that if you log into your router and adjust the wireless settings. You would need to do this from a machine that is already has connectivity or hard wire the laptop to the router. To do this type 192.168.2.1 into the address bar in Internet Explorer of a connected machine. If you have never adjusted this, the password should be blank and you can just proceed.
Navigate over to wireless settings and the security setting and change the passphrase to whatever you want. If there were any wireless computers working on the network previously then you will need to update the passphrase on them to reconnect.
To actually reset the router, there should be a small pinhole button labeled "reset" which you will need to depress for at least 30 seconds, probably more.
The manuals and troubleshooting guides over at
http://www.belkin.com/support/ will probably be very helpful.