Inbox emails disappeared

All of the emails in my inbox have disappeared I did not delete them and they are not in my trash folder.  What is the cause of this and how do I retrieve them?  This is a loss of over 8,000 emails from as far back as 2009!  These are business emails that I am required to maintain a record of.  Please advise how to fix this and retrieve the emails that have been deleted without my consent.