Missing email, urgent!

Missing email, urgent!

Hi,

Two days ago I noticed I am missing all sent email messages from June 14 up until two days ago. New messages I send do get put in the Sent folder but I am missing a lot of messages between June 14 and November 14. I have not archived them, they aren't in the trash folder. I've searched my email client (IMAP) as well as the webmail, they appear to be gone.
There are some really important messages in there which I need to have.
All help appreciated!