I have the same issue. I just noticed my notes app is gone, so are all my notes on it. Frankly I thought my toddler son has done something to it, but then I realized he was sleeping all night!. I tried using few sofwares, but none of them were able to recover my notes. I read all messages here, and reset the home apps, but nothing. You can get the app back by reloading it from app store, but it doesn't return your notes. It is ridiculous that there is only one response to this question from 2016 and it is marked as "resolved", but in reality no one from Apple seems to care to come and write in response to all complaints in 2017, accepting their responsibility, that Apple's new IOS has caused this issue, and provide a solution.
I pay for icloud, but my notes do not get backed up on icloud. Also, the back up on iTunes is very old, as I use the notes on daily basis.
Very upset and angry at apple who can see these complaints and surely know about this problem but prefer to remain indifferent until they have sorted this issue out...all you are doing today is adding to the frustrations of many people.
Please address this mess from your IOS.
thanks