There is no reason why LectureNotes should get uninstalled in the course of an Android system update. The fact that the Google Play app crashes (@ussastro: Does that happen for you as well?) suggests that something went wrong during the Android update, leaving the device in some kind of `weird´ state (with LectureNotes gone being only a visible side-effect).
I agree with muzzy996 that a factory reset might be worth considering.
I understand that I sound like a `know-it-all´ after the damage is done, but please perform regular backups of your notes! Or what will be your recovery strategy if the device breaks down, gets lost, or gets stolen? Even if you do not perform regular backups, perform a backup of important data before installing an Android update.
09-30-2015 12:09 PM