Depending on your version of Outlook, you can recover items from Deleted Items, even if you've emptied it:

• While in Outlook, click on Deleted Items
• On the menu bar, click on Tools - Recover Deleted Items
  
• This will bring up a list of items that have been emptied from your Deleted Items:
  
• To recover an email, select it, and then click the Recover Selected Items button:
  

You can also recover items from Outlook Web Access:

• While in Outlook Web Access, click on the Options button in the lower left hand corner:
  
• Scroll down to the very bottom of the Options page, and you will see the button for recovering deleted items:
  
• Find the message you want to restore, click on it, and then click the Recover button:
  

When you recover a deleted item, it gets restored into Deleted Items. Also, it may take a moment to appear.