The Office 2013 (KB2817630) update yesterday which caused many Outlook 2013 users to lose their folder pane has been pulled from Windows Update.
Read more at:
Microsoft pulls buggy Outlook 2013 update | ZDNet
and
Update for Outlook 2013 breaks folder pane | ZDNet

If you are affected by this, then the tutorial below will show you how to uninstall the (KB2817630) and (KB2817347) updates. A restart will be required.
http://www.vistax64.com/tutorials/92402-windows-update-uninstall.html
http://www.vistax64.com/tutorials/92402-windows-update-uninstall.html