There's another thread on this. Hold on and I'll look. In short, your notes are most likely in your email in the Notes folder. If you don't see a Notes folder, go to settings>Mail, Contacts, Calendars>email account> Toggle OFF Notes. That's OFF. Seems backwards, but the way it works. Then check the email folder for Notes. I'm assuming you meant they vanished from your Notes app.
Sent from from my ancient but trustworthy iPhone 5