Hello errachit

                       In this case if all the lights on the modem are ON you need to connect any device directly to the modem since this is just a cable modem then try accessing internet and if that works fine then there is no issue on the modem and if there is no internet on the internet perform hard reset on the modem.  please, disconnect the coaxial cable from the back of the device? 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 connect the coax cable back and wait until your device fully boots up. 

                      http://arris.force.com/consumers/articles/General_FAQs/T25-Reset-Button