https://www.theregister.co.uk/2019/02/04/microsoft_windows_update_woes/

It has been a trying time for Microsoft punters after a Windows Defender update left some PCs unable to boot last week, while other folk continue to struggle to even get to the update service.

A tweak to Windows Defender slowly creeping its way over PCs has an issue that can stop Windows 10 booting (and a few iterations of Windows Server – Microsoft points to Server 2016, but some users have reported problems with Server 2019).