hard wire to the router and verify you have internet
start - run - cmd
in the dos box type ipconfig /all
look for the default gateway
open internet explorer and type in the default gateway number from above (it goes right were you would type www.google.com)
it should prompt you for a password. Most linksys are no username and admin as the password by default, or whateer you changed it to when it was setup. If this doesn't work you will need to reset the router (read as loose all settings and start over on your router)
if you do get logged in go to the wireless tab then security and write down your key or change it to something you will then know.
A remote possiblilty is to auto configure the linksys. This can be done if you have a linksys wireless card and a newer linksys modem - you'll have to look on linksys site for directions on that, I have never actually done it.