Yesterday Microsoft released a few updates for Windows & Office. One of the updates, KB2817630, appears to break Outlook 2013. The screenshot to the right is what happened to one of our customers after the update was automatically installed. Absolutely no folders appear in the folder pane.
At this point it appears the fix is to install this hotfix available from Microsoft. Update: Hotfix has been pulled for some reason.
Update: Microsoft has published a Technet article detailing this problem with some resolutions.
Update: Microsoft has now published an Outlook 2013 Update to fix this.