In general, unsaved received messages that you have read will be automatically deleted after 7 days. Unread messages will be deleted from your profile after 2 weeks. At the same time they will be forwarded to your email address, provided you have verified it.
If a message is not present in your profile it is also not present on our servers because we keep, at all time, a faithful copy of your "live" profile.
I am sorry but we can't help you retrieving your lost messages.