Manually recreate the Show Desktop icon
Create a text file with the extension .scf (preferably Show Desktop.scf). SCF stands for Shell Command File. SCF files are executed by shell32.dll and the file extension is one of the special ones that remain hidden even if you instruct Windows to show file extensions. Paste the following code into the file.
[Shell]
Command=2
IconFile=explorer.exe,3
[Taskbar]
Command=ToggleDesktop
Save the file and place the same in Quick Launch bar and now you’re done.
Using the Regsvr32 command
This one is pretty simple, just go to run prompt or command prompt and type in the following command.
regsvr32 /n /i:U shell32.dll
The regsvr32 command line tool used to register .dll as command components into the registry. The above command with instruct Windows to register shell32.dll with optional install string, which enables Show Desktop icon back into place.
The Show Desktop icon file should be available now automatically in Quick Launch bar after executing the regsvr32 command.