Are you talking about the stock Notes app?
I've never seen this error, but my guess is you have your Notes set to sync with your email provider. You can see this in your account settings. This copies all your notes to a folder in your mail account. If your email were unavailable for some reason, you would probably get an error of this type when you create a new note. It should correct itself as soon as it finds the mail server again.
Check your email account on the server and see if you have Notes folder, and check to see if your notes are there.
Assuming everything I've guessed is true, I would only worry if the problem persists and your notes are never get copied to the folder. Mail services often have momentary glitches, and they usually correct themselves.