I don't know how to restore the emails but possible reason why this might have happened are.
- If they disappeared from on line web mail this might have occurred if a email client is set up as a pop account. By default most email clients configured as pop accounts are set to remove emails from the server after a certain period of time 14 or 7 days.
- If they disappeared from your email account and it is set up as a IMAP account it could be due to account being configured only to synchronise last 7 days of emails.