On May 11, 2021 around midday Microsoft rolled out some sort of patch, that basically made Windows Outlook 365 desktop clients unusable.  Trying to view and create emails became impossible as you type and hit enter the text would disappear. 

Office are scrambling to get Outlook Desktop versions working and many have restored to the previous version prior to the update. 

You can run this command and it should download and install the previous version.  

C:\Program Files\Common Files\microsoft shared\ClickToRun\OfficeC2RClient.exe /update user updatetoversion=16.0.13901.20462

Alternatively some users have been able to use Outlook in safe mode and that also appears to work.

Outlook.exe /safe

The question will be swirling for days as to how this sort of patch could be released outside of the updates and without any level of QC and testing.

It could be hours before the update is received by users.