There are many factors that may cause you lose iPhone notes. For example, you disable the notes in some email account settings, deleted some emails, deleted notes on other devices with the same iCloud account and iCloud sync enabled, etc. If you met with this situation, it's highly recommended to check your email settings in Settings app, and then confirm if you've deleted some email accounts or got password changed from your iPhone, and check if you've enabled your iCloud. If you still couldn't get back the notes, you can try to scan your iPhone with this iPhone notes recovery tool, it will help you recover the lost note and transfer it directly back to your iPhone.Alas. Notes is off. I've lost them. It'd be nice to know what I did to lose them, though.