MacBook says it is not charging when magsafe adapter is plugged in

When the charger is disconnected from the macbook, it flashes green. When it is plugged in, there is no light and the onboard indicator says NOT CHARGING. I have tried plugging the adapter into every outlet, but no such luck. I even replaced it with the extension cord and still nothing. It was just working this morning at a friend's place but will not work in my dorm room. Anyone have any ideas?