In your preferences folder there is a file which contains a cache of the pref panes. Sometimes this gets messed up and causes panes to disappear or to end up showing multiple occurrences of some pains...
The file is called "com.apple.preferencepanes.cache" and is in your user folder under ~/Library/Caches/
It also *may* be in your ~/Library/Preferences folder (I *think*) if you have an older version (I could be wrong about this one though)...
Delete this file and re-open preferences, and everything will show again...