If an app seems to have vanished, try searching for it. From the launchpad (the screen full of app icons and/or folders), swipe down from the middle of the screen. Type in "Notes". The Notes app cannot be deleted, so it must be there somewhere. When it's found it'll be listed in the "APPLICATIONS" section. Not only can you jump straight into the app from there, but just as an FYI it'll tell you what folder it's in, if it's in a folder.
If you know an app should be in a folder, but you don't see it there, remember that folders can now be multiple pages. Maybe it's hiding on one of the other pages. (Or, it could be hiding in plain sight. Apple keeps changing app icons, and more than once I've been unable to see an app that's right there, because I'm looking for its old icon.)
Tying a note to an email account allows it to be synchronized across devices. The association between a note and an email account is established when the note is created, and cannot be changed. (Although you can, of course, copy a note and paste it into a new note associated with a different account.)
On an iOS device there is an artificial account called "On My iPhone" (or "iPad") that is for notes that are not associated with any email account and therefore are not shared across devices. There ought to be an equivalent on the Mac, but I can't seem to find it.
On each device, you can configure which of your email accounts will play this note-passing game. On an iOS device, go to Settings→Mail, Contacts, Calendars. On a Mac, go to System Preferences→Internet accounts. In either place, select one of your accounts and turn "Notes" on or off. All devices that turn on "Notes" for a given account will automatically share all notes associated with that account.
The email account is only a conduit for passing the notes. They actually show up in the Notes app on the various devices.
In Notes on iOS, if you keep moving left, you'll come to the top-level menu, which is a list that looks like:
- All Notes
- On My iPhone
- one of your email accounts
- another of your email accounts
- ... and so on
Selecting "All Notes" does just that: it leads to a list of all of your notes no matter which email account they're associated with. Each of the other entries leads to a list of only those notes associated with the chosen account.
If you're looking at one of these lists of notes and tap "New" in the upper right, you will create a new note associated with that email address. If you tap "New" while looking at "All Notes" the account for the new note is whatever you selected under Settings→Notes→Default Account.
If you created a note and it disappeared mysteriously, one of three things happened. The most likely is that it was created with one account but you're looking at the list for a different account. Almost as likely is that when you finished editing the note you neglected to tap "Done". That leaves the text vulnerable to an accidental deletion. Think of "Done" as a synonym for "Save"; if the battery dies before you tap "Done" you may lose your work. The third possibility is that you deleted the note on one of the other devices it's synched with.
