Conflicts between antivirus software and the recent Microsoft April 2019 Patch Tuesday updates are causing Windows 7, Windows 8.1 and Windows Server 2012 R2 to freeze, be unable to boot, or hang on installing updates. Microsoft has placed a block on the conflicting updates so that they are not offered to users running Sophos Endpoint until a solution is made available. Microsoft is aware of the issue and has added the known issue to the knowledge base articles for affected updates. Windows 10 may also be affected as well as Avast and Avira products.
Source: https://www.bleepingcomputer.com/news/microsoft/microsofts-april-2019-updates-are-causing-windows-to-freeze/