While I read elsewhere to clear recover cache, I think I found an easier answer. For me, all I did was:
1 - press the down-volume key until it was on vibrate
2 - press the down-volume key one more time to activate the "Alarms Only" mode (a new feature in Marshmallow)
This step seems to "unconfused" the system, putting into a state it knows how to deal with
3 - press the volume key yet one more time to bring up the alarms only view a press "end now" button.
(This time it works ... whereas the "total silence" end now did not work)
You system should now be ready to use. Turn volume up/down and turn on/off the "do-not-disturb" settings as you see fit!