It’s easy to accidently delete a Distribution List - clicking the Delete button deletes it, not the selected members of the DL. To remove members from the DL, you need to click the Remove button.

The deleted DL will be in Outlook’s Deleted items folder. If you have a lot of items in your Deleted items folder, the easiest way to find it is by searching for it by name or sorting by icons and browse. Once you find it, drag it back to the Contacts folder.

Screenshots: Remove Members, Don't Delete