I noticed that Ubuntu would not show desktop after upgrading from 18.04.5 LTS to 20.04.2 LTS?

I think it was unable to upgrade the NVIDIA graphic card or placing the file in the appropriate directory.

How I resolved it:-
I resolved the issue by running

sudo apt dist-upgrade

I think this automatically install / update / repair all the needed packages for the current version.

Just to add - after upgrading to 20.04.2 LTS and decided to reboot, that was when the desktop wouldn't show - it got stuck on blackscreen.
to be able to open the terminal from this stage
Just press on Ctrl and Shift and F1 or F2 to toggle to terminal screen where you can put in your username and password --
Then use sudo -i
Then put your username and password again
Then run -- sudo apt dist-upgrade

Once completed - then type Reboot and then click on Enter

Boom you are done and everything should load properly