System Preferences icons lost/gone all the same
EDIT : My link concern an older version of Mac OS so you will not find all the files.
I updated my answer to match with my version of Mac OS (10.7.3).

It's seems that this person had the same problem and found a solution on this site.
Try the following and let us know what happens : 


  Quit System Preferences if it's running and trash the following files (make a copy of these files on your desktop for backup purpose) :
  
  /Users/yourusername/Library/Preferences/com.apple.systempreferences.plist
  /Users/yourusername/Library/Caches/com.apple.preferencepanes.cache
  
  Then log out/in or restart and check System Preferences for proper
  functionality.


You can access your library by pressing the CMD key when clicking on the Go menu of the Finder.

And about the reason of that change it can be possible that one of the setting file of the system preference pane has been corrupted. That's why you can try to trash settings file and see if the problem is gone.

Hope this helps !