Have you checked in "Settings" and then "Sound" to make sure it isn't muted? You may also want to try restarting (turning off and back on) the phone to see if this resolves it. If it doesn't, and it isn't muted or turned all the way down in sounds then try starting the phone in "Safe Mode" and see if it will work in there. If it does, then you know there is something (virus, malware, bad app, etc.) that is on the phone and interfering. If it doesn't work even in "Safe Mode" then you likely need to have it fixed (hardware problem).
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.