Missing notification bar and navigation bar on an Amazon Fire HD 10 tablet I turned on my Amazon Fire HD 10 tablet the other day, and some (but not all) of the bottom navigation bar buttons had disappeared.  I rebooted, hoping that would clear up whatever caused it, but now all of my navigation bar buttons are gone.  My notification bar is gone too, and I am unable to swipe down to activate the notification shade.  There is empty black space where the notification and navigation bars are supposed to be.

If it matters, I have the Google Play Services and the Google Play Store side-loaded, through which I installed typical Google apps.

(I'm asking and answering this myself mostly so that if/when this problem recurs, I'll be able to remember/find how I fixed it.)
I don't know exactly what caused it, but it seems related to Gmail notifications.  Going to Settings > Apps &amp; Games > Manage All Applications, choosing Gmail, and unchecking the "Show notifications" checkbox caused my notification and navigation bar to return.  Re-enabling "Show notifications" did not cause the problem to recur.