Microsoft told BleepingComputer that the issue is caused by users not fully deploying the latest SHA-2 enablement packages. Windows Server 2008 servers will no longer boot if prerequisites aren’t installed before applying the out-of-band KB4539602 update released by Microsoft on February 7 to patch a wallpaper bug. Windows admins came up with two unofficial fixes to revive Windows server servers that won’t boot after the update. Windows 7, Server 2008, and 2008 R2 reached their end of support on January 14, 2020, and will not longer receive software updates, security updates or fixes.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-server-2008-servers-don-t-boot-after-kb4539602-update/