No idea what email client you are using, did you look in your Recycle Bin & your ISP's web page.
You are supposed to backup your emails regularly, then retrieving them is much easier. The deleted emails are still on your hard drive somewhere, although they will not be visible to you or any ordinary search tool. File recovery software may help: http://pcsupport.about.com/od/filer...
Can you find them in the trash?