Ringtone and vibration not working on Nexus 5 (Android 5.0.1)
I had this problem and it was so annoying, because I missed all the important call due to this error or setting. I didn't change this setting and only Nexus phone know how it changed this setting.

Please follow the steps below and it will start ringing.


Press Volume Up key on the the phone.
Select the option "Indefinitely"
Select the Settings icon (the wheel icon)
Under "Priority interruptions" section, enable Calls.


Enjoy, no more missing calls