Outlook went down for four hours Wednesday. What happened?

At mid-morning Wednesday in the U.S., Outlook stopped working. Dead in its tracks. Both the Click-to-Run version of Office (er, Microsoft) 365 and the installed (MSI) version of Outlook refused to start: 

Today when I went to check my email, Outlook would not open; it would load the “Starting Outlook…” splash screen, which would close without opening the Outlook window itself, and the taskbar icon went away. Looking in Reliability History, it states that Outlook has crashed.

To read this article in full, please click here