So there are basically two reasons why the notes disappeared.
1. Users accidentally deleted the Notes folder in Gmail.
--> In this case, the notes will be stored in Trash for another 30 days. Users can go to their Trash folder and retrieve back the deleted note within 30 days, however, if no action was taken, the notes will be deleted after 30 days.
2. According to this article http://gmail-miscellany.blogspot.sg/2013/08/gmail-apple-notes-app.html, the way the Notes application on iPhone communicates with Gmail is that when a user creates a note using the application, it will send it to the user's designated email account in a form of email with a label "Notes." When this happens, the iPhone does not store the physical copies, what it does, instead, is syncing to the Notes folder in the designated email account when the user wants to access to certain notes. This "Notes" label works just like any other labels. If an email is labelled as "Inbox," it will appear in inbox, and if it is not, this email will be stored in a place called "All Mail." And thus, when the Notes folder in Gmail is empty (because the emails were unlabelled,) the Notes application has nothing to sync/ access.
--> This was what happened in my case. So the solution for this is
a. Go to All Mail
b. Select the notes you created (hopefully you still remember what notes you have created)
c. Label them as Notes
And if the user's iPhone is connected to the internet, the Notes application will be able to sync the notes back on the phone.