I'm not sure what you mean when you say that the AirPort Extreme is not working. If the green status light is on, the AirPort Extreme is working.
Are you saying that Wi-Fi devices cannot connect to the AirPort Extreme Wi-Fi network?
Or, if they can connect, can Wi-Fi devices get a good Internet connection.
The first basic troubleshooting step would be to power off both the modem and the AirPort Extreme.
Wait a minute, then start the modem first and let it run for a full minute
Then, start up the AirPort Extreme and let it run for a full minute
Check to see if things are working