Are you using an Exchange account or is everything kept in a pst-file?
If you are using Exchange, then you could try the Tools-> Recover Deleted
Items... function to see if it is still kept in the Exchange retention
folder (if enabled).
If not, or if you are using a pst-file, the deletion is quite definitive and
a restore from backup is needed.
If you don't have a backup with that data in it, then as a last resort you
could try the following;
http://www.howto-outlook.com/howto/restoredeleteditemsfromanoutlookpst.htm
