After the September updates were released from Microsoft a bug has been found to make the folder pane (on the left hand side) disappear.
I actually experienced this for myself today. Here is a link from Microsoft with a detailed description and fix. MS are working on re-publishing the September update with the correct versions of mso.dll and outlook.exe to ensure users with auto updates will receive the correct fix.
How to fix affected installations (From MS):
Two updates can get a user into this state. Installing the September Public Update delivers an updated version of mso.dll without updating outlook.exe, resulting in the incorrect user interface.
If you have Automatic Updates enabled, visit the Add Remove Programs feature of your Windows Installation, and uninstall KB2817630. Close Outlook and restart.
If you have installed the August Cumulative update (which you must do manually), removing KB2817347 will correct the issue. From the Add Remove Programs feature, select KB2817347 from the list and select “Uninstall.” Close Outlook and restart.
If you have BOTH Updates installed, the problem is not evident. The issue only manifests when one of the updates has been installed. If you have updated to the September Public Update and you want to roll forward, install the August Cumulative update, KB2817347.