My Applications are not showing after upgrading from Ubuntu 18.04 to 20.04

In my case, this was being caused by an extension. In my particular case once I deactivated the Appfolders management extension which I had included in 18.04 LTS and restarted the computer everything worked perfectly.

Here's how to do that if you can't access your applications via the Launcher:

1. Press Ctrl+Alt+T on your keyboard to open a Terminal window.
2. In the Terminal, type gnome-tweaks and hit Enter. This will launch the GNOME Tweaks application.
3. In GNOME Tweaks, click Extensions in the left-hand pane.
4. Deactivate individual extensions by clicking the switch next to them. Alternatively, you can use the switch at the top of the window to deactivate all extensions at once.
5. When you have deactivated the extensions, you need to restart your shell. Press Alt+F2 to open a 'Run a Command' dialog.
6. In the 'Run a Command' dialog, type r and press Enter. This will restart the shell.
7. Open your launcher to see if the problem is fixed. Use these steps and the process of elimination to determine if an extension is causing the bugged behavior.

I ran into the same issue even though my extensions were disabled. It turned out to be caused by having the new theme - Yaru (previously called Communitheme) - installed on 18.04 before upgrade. After switching from Communitheme to Yaru in Gnome tweak tool and from "Ubuntu Communitheme" to "Ubuntu" during login under gear icon, the issue went away.

I disabled all GNOME extensions as suggested by pomsky and after a restart of GNOME-shell the applications were shown again in the dash! Afterwards I reactived one GNOME extension after the other and finally, when I came to the appfolder-management extension, the applications were gone again. So I deactivated the appfolder-management extension again and everything were fine again.

So I can confirm, that there's a problem with the appfolder-management extension but to be safe you have to disable ALL extensions first and then reactivate them EXCEPT the appfolder-managment extension. Furthermore, after GNOME shell 3.33 there's no need for that extension, because after 3.33, GNOME Shell handles this feature natively.