One of two places to look. Either you have Mute turned on or Settings > General > Sounds has the volume turned down, or not ringtone set.
Mute is either set with the switch next to the volume rockers, or in the multitask bar. Which it is is determined by how you've got Settings > General > Use Side Switch to: set. The multitask bar will have the opposite control that you set the switch to. To get to the multitask bar settings, double tap on the Home button, then slide the icons to the right to reveal volume, play, brightness, and the rotation lock or mute control.
Mute only affects system sounds and those apps that choose to honor it. Most media apps like Music or Videos ignore it.