First make sure your volume button isn't stuck. That can cause this. Then go into "Settings" and then "Volume" and make sure that all the volume levels are set high enough. Then make sure that the ringtone you have chosen is actually working. You may also want to try setting it to a ringtone that came with the phone, rather than one you added. Then try restarting (turn it off and back on) the phone.
If none of this helps then try starting the phone in "Safe Mode" and see if it will ring in there. If if does ring in there, then there is something on the device causing this, virus, malware, bad app, etc. That you will have to remove.
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.