First step is to connect your Mac directly to the modem using the same Ethernet cable that is connected to the AirPort Extreme.....to make sure that the Mac can connect to the Internet that way. If it can, that tells you that the modem is working correctly, the Ethernet cable is working correctly, and we can move to troubleshooting the AirPort Extreme.
However, if your Mac cannot connect to the Internet when it connects directly to the modem using an Ethernet cable, then the first task will be to try to troubleshoot the modem. Obviously, if the modem is not working correctly, then nothing else will be working either.
We'll need more information to be able to start the process. Please answer the questions below:
1) What is the make and model number of the modem?
2) Was the AirPort Extreme working before, and this problem has just started, or do you have a new AirPort Extreme that you are trying to set up for the first time?
3) We assume that you have a Mac, correct?
4) What troubleshooting steps have you already tried?