Folder icons missing on Desktop
First of all a quick reboot might fix your problems. If that does not work, you could try relaunching finder.  Apple icon → Force Quit → Finder → Relaunch. If that does not work you could try deleting the Icon Cache. Go to Terminal, which you can just search for, or go to Applications → Utilities → Terminal. Paste exactly these commands in one at a time. 

sudo find /private/var/folders/ -name com.apple.dock.iconcache -exec rm {} \;

sudo find /private/var/folders/ -name com.apple.iconservices -exec rm -rf {} \;

sudo rm -rf /Library/Caches/com.apple.iconservices.store


Do note that when you put in a sudo command, you must put in your password once. The characters do not appear when typing them, so don't worry. 

If this does not work, you should do a complete restore of the Mac. Look up tutorials online, but make sure to do a backup of your Mac with Time Machine or a cloud based service.

Good luck!