The good news: In all likelihood, the data files aren't really missing. When Windows 8 upgrades your computer, it creates a folder called "windows.old" and it contains all of the files used by the previous Windows installation, including your user account files.
To recover your data files (and other personal files) open Windows Explorer and go to C:Windows.old\Users\username\AppData\Local\Microsoft\Outlook. Your old data files should be there, or in the Outlook Files folder in My Documents (the old My Documents!).
Because the folders under the AppData folder are usually marked hidden, you may need to configure Windows Explorer to display hidden files and folders or remove the read only flag from the properties of the AppData folder and apply it to all child folders.