The email accounts on your iPhone can store notes as well in addition to just sending and receiving emails. Sometimes when notes disappear, actually they haven’t been deleted but still live on a server tied to your email address. And the reason why you cannot see them in the Notes app is because something goes wrong with your email address - you recently deleted an email address from your iPhone, to name just one example. In a situation like this, you have to set up your email account again to recover the notes.