Black Screen on Upgrading from Windows 10 to Windows 11

[Nits58]

Hi! This is my first thread in this forum. I encountered a problem on upgrading my Windows 10 to Windows 11 as suggested from the Update and Security section. The installation downloaded and installed. Now the PC does not boot into Windows 11, but I get a black screen instead. I am even unable to roll back to windows 10. In the BIOS I cant find the Windows Boot Manager option. How do I either load windows 11 or roll back to my original installation. Please help.