If you read the manual, it will explain how to reset your router, and return it to default settings (including the default password). On Linksys it usually involves pressing the reset button, and rebooting the router while holding said button in. But check your manual first. If you don't have your manual, Linksys online will have it for you to read.
You'll unfortunately need to set it all up again, so make sure the first thing you do is change your SSID (wireless broadcast name), and setup security on your wireless.