Try going into "Settings" and then "Sound". In there check all the volume settings to make sure they are set to be up high. Then check the ringtone you have set to make sure it is working correctly. If by chance you deleted the ringtone or it was somehow removed, then you could have no sound.
Should all that not help, then check to see if you get any sound at all on the phone. If sounds in general work, and the ringtones can be heard when you test them, then there may be a problem with the volume button.