How to fix my att uverse cable?i got a new computer so i was hooking it up and my screen went black!?

How to fix my att u-verse cable!i got a new computer so i was hooking it up and my screen went black how do i fix this!please help!!!|||On the back of the Residential Gateway (modem-router) there are four ethernet ports for connecting computers. Connect the first port to the ethernet port on your computer. Make sure the connectors are properly seated.