Issue:

The Reading Pane in Outlook 2013/2016 suddenly stops displaying email content and instead shows nothing.

Resolution:
In my instance the problem was resolved by disabling Hardware Graphics Acceleration in Microsoft Outlook. To do this, follow the below instructions:

1) In Outlook, Click File> Options
2) In Outlook Options, Click Advanced
3) Scroll Down to the ‘Display’ Settings
4) Check the box for ‘Disable Hardware Graphics Acceleration’
5) Restart Outlook for it to take affect

See below for reference: