I upgraded from Ubuntu 18.04 to 20.04 but desktop files and icons are missing, How do I should solve this problem?
Try switching desktop manager.
I also just upgraded my stable 18.04 to 20.04. Initially I can't even see my initial login prompt and I have forgotten that I installed lightdm and made that the default vice gdm (gnome desktop manager) which is version 3 now (gdm3).
I followed this nice how-to: Switch Display Manager in Ubuntu 20.04
You can have both gdm3 and lightdm installed but one can only be set active.