The Windows 10 KB4549951 cumulative update is reportedly failing to install and is causing blue screens of death (BSOD) after installation reboots, among other issues, according to user reports. Microsoft says that they are not currently aware of any issues with the update. Users have also reported problems with their Windows 10 devices being unable to boot again after installing the update on its own. KB454951 is a cumulative update with security fixes released as part of this April 2020 Patch Tuesday.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10-kb4549951-update-fails-to-install-causes-bsods/