To restore deleted emails, check to make sure the emails are not still in your Trash folder or are hidden. If that is not successful, submit a restore request to Yahoo support.
Only emails deleted within the past 7 days can be restored. To submit a restore request, login to your Yahoo account and go to the help center. From there, locate the Restore Request form, provide the relevant information and submit. Do not submit another request while the first is being processed, as only the most recent request will be accepted.
It is possible that the support team will not be able to restore the emails, and it is always a good idea to back up any important documents on a regular basis.