First restart your computer (many issues are solved with a reboot)
After you reboot try to go to a web site (like www.google.com)
If you still can not get on the internet look at the bottom right of the screen and look for an icon that looks like a computer monitor
If there is a red "X" over the monitor or a yellow warning triangle then check to see if your green cable is connected to the back of the computer and to the outlet on the wall
