There are two ways you can recover the notes: by restoring from a backup, or by having synched them to Outlook before they were lost. If you have no backup and did not sync them to Outlook, they are irretrievably lost.
It is very unusual to hear of notes being lost as they are kept in device memory, however, it is possible that a memory problem could cause them to be deleted. What is your Options / Status / File Free amount?
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