A common cause is that you have not plugged your telephone line into your modem. The second most common problem is plugging your telephone line into the wrong jack on your modem.
Many modems have two jacks, one for the telephone line and one for a telephone handset.
Double check to make sure that all other devices are hung up properly. There may be a problem on your telephone line. My modem dials and rings with no answer. First of all check to make sure that you have dialed the correct phone number. It is also possible that all the circuits to your ISP are in use.