Which one ?
The password to your ISP ?
The password to your e-mail ?
The password into the setup pages of your router ?
The wireless security passphrase ?
I'm guessing, the latter. Try connecting the computer and router via an ethernet cable and open the setup pages -- under wireless setting you should find the wireless passphrase.
If all this means nothing, suggest you consult the quick start leaflet which came with the router or download a full manual from the manufacturer's website.