If you accidentally delete a search folder, the folder is gone and cannot be "undeleted". You'll need to recreate the search folder.
- Scroll down to the end of the folder list to the "Search Folders" folder.
- Click on Search Folders
- Choose New Search folder from the menu
- Select the type of Search folder from the predefined list.
- Click OK to go back to Outlook.
To add it back to your Favorite Folder list, drag it up to the Favorites list or right click and choose Add to Favorites.
Choose Create custom search folder from the end of the list to create a folder using your own criteria.