Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Do you mean the bar at the top of the screen? If you do then I don't know how you have done that :confused:
 
Possible "undo"

No matter how you did it, system user interface preferences are stored in com.apple.systemui.com (I think that's the full name) which you will find in HD/Users/YourUserName/Library/Preferences

Move the plist file (not com.apple.system.com - it has to reference the user interface or "ui") to your Desktop
Restart. This will create a new plist file in Preferences.
If all is well, you can Trash the ui plist file you placed on the Desktop.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.