@WinniWoo: Further to the above, if you open the Messages app, tap the 3 dots in the top right and select 'Recycle bin' then any deleted messages should be available here. To recover a message, press and hold on it, tap on any others to select them (or hit 'All' in the top left) and tap 'Restore'.
It may be the case that the messages were deleted to make way for new messages. In this instance, tap the 3 dots in the top right of the Messages app > Settings > More settings > Delete old messages, and toggle this option off.
