In this case you can try on different web browser to access GUI of the modem, using different device connected to modem Wi-Fi and also you can try after clearing the browsing history, cookies and catches on the browser then give it a try if still you are unable to access GUI then the only option is to reset modem to use the default login credentials to access GUI.

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