I had a similar freak out incidence couple of years ago where my notes and contacts just disappeared. First thing I would suggest is going to Settings> iCloud> and turn on the notes toggle. Maybe you've synced your notes and just in case you never have, I'll suggest you start syncing them to iCloud for future so that you never lose them. Secondly, I would suggest going to Setting> Mails, Contacts, Calendars> then click on each individual mail account and tinker with the notes toggle. Maybe you've synced them with one of your accounts and now it's just turned off. This did the trick for me.