If you don't have access to any computers that are configured to use the wireless, and therefore can't look up the password, there's another way to do it if you have physical access to the router.
First, reset it to factory settings, usually with a tiny button on that back that you have to press with a pen or paper clip. Once it reboots, the wireless is usually enabled without a password. If it isn't, or if you want to password-protect it again, connect your laptop directly to the router with an ethernet cable and access the router's admin interface with your web browser. Usually it's at http://192.168.0.1 or http://192.168.1.1, but check the manual to see what IP the router takes by default. It will prompt you for a username and password, which is usually "admin" and an empty password, but again check the manual. I've also seen routers where all of this information is on a sticker next to the reset button.
Once you're logged in, you can enable the wireless network if it wasn't already enabled, give it a name, and enable the wireless security. Use WPA2 security, not WEP or WPA (both are weak and crackable). Use something like Password Safe to generate a passphrase and store it securely so that you can look up later.