Check 2 things:
First...look above the volume buttons. This is your silencer switch. Make sure that the switch in pushed towards the screen. In that position, all sounds are on. When the switch is pushed towards the back all sounds will be silenced except for alarms.
Second......If the switch is in the correct position, check your Do Not Disturb. From your home screen slide your finger from the bottom to the top. The control center will open and if the half moon symbol is highlighted white, tap it and DND will be turned off.
If these do not fix the issue then you should make an appointment and visit an Apple store.