Why are my emails disappearing from MacMail?

I bought a MacBook Pro in September 2012. Have just noticed that all my emails from the 23/12/2012 back are gone? I have an IMAP account and can't see anything in Advanced Settings that would cause this to happen. How do I retrieve my (not so) old emails? This is a business email so a major pain in the ***. 😟 Any help would be appreciated. Thank you.