If you have changed the login credentials previously then it will not accept the default login credentials to login the modem interface or GUI so to access modem interface using default credentials you need to perform hard reset on the modem. 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.  At the back of the modem, in the middle section you will see a small hole, inside that hole there is a button, and that is the reset button, please insert a paper clip or a tip of a pen and hold it for 15-30 seconds until all your lights go off, afterwards wait until your device fully boots up.

http://arris.force.com/consumers/articles/General_FAQs/SBG10-Web-Manager-Access