You can reset the router to factory settings if your password has changed. There should be a reset button on it somewhere, just hold it for about 30 seconds and you should be able to use the default password again.
If you have an internet connection that requires a password, make sure you know what it is before you do this though so you can configure the router again.
Oh and I think the default Linksys username is blank (at least mine is), so try that first.