If you are unable to login to the modem GUI you can type word password in password field in lowercase and try logging in. If still you are unable to login you need to reset modem since there is no other option to reset or forgot password. So after resetting you can setup the login credentials again. But after resetting the modem will go to default setup any custom settings will be removed so use the default network name and password  to connect to your network. To reset modem you need to press and hold the reset button the modem for 15-30 secs then after modem reboots completely connect any device to the modem default wifi network using the wifi credentials mentioned on the modem label and follow the steps mentioned on the below link to access modem GUI or the web interface.

http://arris.force.com/consumers/articles/General_FAQs/SVG2482AC-First-Time-Installation