Default Ringtone automatically resetting to "Silent"

I use a Samsung Galaxy Note 3 (with Android 4.4.2), which was working fine. Recently, the device started resetting the default ringtone to "Silent", no matter what ringtone I set it to. Because of this, when somebody calls, we cannot hear the ring (but can see the incoming call details on the screen). I have reset the ringtone of most of my contacts to something other than default. But this is an issue when somebody who is not in my contact list calls. Any idea why this is happening ?