Does it ask you for the 10 digit system key (which is the wireless WEP encryption code) or is it giving you a 20 digit number? If it's giving you a 20 digit number you'll have to call them.
What you can do, though, is if you go to that site and run the link with a password you do want to use, it should work again. If you don't want to do that, then the other way is by pressing and holding the reset button for 30 seconds to factory reset the modem.
