An issue is being reported widely today that a specific update for Microsoft Office 2013 causes the Folder Pane in Outlook 2013 to show an empty display. Among a few Office 2013 releases, KB2817630 appears to be the culprit.
Uninstalling KB2817630 seems to resolve the issue. There’s no word yet that Microsoft has acknowledged the problem, or is working on a fixed update.
There’s a current threaded discussion going on in the Office forums on the Microsoft site here: Outlook 2013: Folder Pane is empty