|"all mails in this folder are very important for me"|
You never thought to back them up, even though they are very important to you? Words fail me.
Have you looked in the "Deleted Items" folder in Outlook or did you delete them from there as well? if you did, try this: http://www.recovermyemail.com/
I hope you recover all or most of them, and in future back them up on a regular basis!