This is in the Help.
Restore the Unread Mail folder

The Unread Mail folder is a default Outlook search folder. If deleted, you can create it again.

1. In Mail, on the Folder tab, in the New group, click New Search Folder.

Keyboard shortcut To create a new Search Folder, press CTRL+SHIFT+P.

1. In the Reading Mail section, click Unread Mail.
2. Click OK.

Note Use these same steps to restore any default Search Folder. In step 2, in the New Search Folder dialog box, click the Search Folder that you want to add.