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.)