I read on some blog, that the problem stems from a bug relating to the "Smart Sensor Settings" (i.e. Double-Tap to silence and Smart Profile: Face Down to Vibrate). If you enable these settings, it for some reason silences your ringing for good. The only way to get your ringtones back, is to reset phone back to manufacturer settings, and setup your phone againg, by loging into google account. So once you have your ringer back, make sure you don't enable the "Smart Sensor Settings" which are under your "Sound & Display Settings".