If you've spent time assembling a distribution list in Outlook, then accidentally deleting it with an unplanned keystroke can seem like a major catastrophe. However, fixing the problem is easier than you might think.
The Microsoft Crabby Office Lady blog points out that provided you go immediately to the Deleted Items folder within your Inbox after losing a distribution list, you can drag the just-deleted list back onto the Contacts folder to retrieve it.
In fact, this logic works with any item in Outlook; while the Deleted Items folder is normally dominated by email and accessed through the Mail view, it will also contain anything else deleted within Outlook itself. To control how often the folder is emptied, head to the Other tab on the Tools --> Options dialog, where you can set your system to automatically empty deleted items whenever you exit Outlook.
Recover a mistakenly deleted distribution list [Crabby Office Lady]