Thanks again every one! Finally able to start using Trados Studio 2022 again! 

I tried .net repair and it says .net 4.8 is already installed but unable to start Trados Studio 2022 (and other older versions) with an error saying 4.8 is needed. On my Windows 10, it was also impossible to uninstall Trados applications (with the same .Net Framework error). Restoring Windows 10 back to the pre-issue status using the backup files dated June this year but got the same error.

As a last resort, I have upgraded to Windows 11 and Trados started work without problem! My machine is quite old and originally running on Windows 7 so no simple task to upgrade from Windows 10 to 11, taking so much time and effort. I really don't know what the cause of this problem, though...  

Sorry for not replying earlier.

The problem seems to be with how .net4.8 is installed on your computer and registered with windows.
I see that an upgrade to windows 11 has fixed this issue.