This generally means that the connection of the modem to the phone system in the building has been compromised, or there is a problem with the way the modem is configured. It is usually something simple like an incorrectly connected phone cord or something of that nature. If you don't hear a dial tone, you have a problem with your phone line. Call your local phone company's repair service. Check the phone cord where it goes into the modem's phone jack and make sure it is plugged into the correct one.
Most modems have two jacks, right next to each other.
Make sure that the cord is not disconnected. Make sure the other end of the cord goes into the wall jack properly. A simple test is to disconnect the phone cord from the modem and plug it into a regular phone and see if you can hear the dial tone. If you cannot hear it, but another phone jack in another room has a dial tone, there may be a wiring problem with this particular jack. Make sure the speaker for the modem is turned on and listen carefully for any sign of problems that you might be able to hear coming from the modem.