This happened to me, and some other folks. Look for a thread asking about how to view/run settings.
Anyway, some fixes mentioned were to turn off notifications for Gmail - hard to do if you can't get to settings, though.
What worked for me was to try and side-load an application - copy an APK file to the 'downloads' folder on your KFHD via a connected computer using the windows file explorer, or ADB - and then install the APK. In my case by clicking on it using a file manager. The KFHD told me that I would have to enable foreign application installs to finish the install of the side-loaded APK, and took me to that portion of settings that dealt with that. In that section was the 'Reset to Factory Defaults' button.
I pressed the reset button, and got everything back. Earlier applications (google play store) that I hadn't un-installed in an attempt to fix the problem were still there. Super User was intact. I downloaded & installed everything again, and all is working as it should be.
Until Amazon comes clean with this walking ad-server, details on how to modify it without introducing issues are going to be sketchy . It's just our lot in life, if you'll pardon. I appreciate having super user access, and all the Google apps I love to use, but what's really required is the ability to modify the boot loader. Until that's available, it's going to be try until it fails, and then scramble to recover.
Nice HW, but limited by Amazon's vision of the customers.