Hi there,
Don't panic, you haven't damaged anything. In the Finder choose Preferences from the Finder menu. In the General tab you will find the top section "Show these items on the desktop:" and the first check box is "Hard disks". Check that box and watch your desktop as the icon reappears.
It is likely that you inadvertently unchecked this while poking around in your Mac's innards.
Explore the other tabs in the Finder Preferences to further customize you Finder experience but don't forgot how you got there if you want to reverse any setting you changed.
If this has been helpful, don't forgot to click the "Thumbs Up" icon.
George