Are you referring to the whole panel, or just the part of the panel that shows opened windows?
If you're talking about the whole panel, try to check if kicker is running (press Ctrl+Esc to bring up the Processes Table). If it isn't running, press Alt+F2 and type in "kicker".
If you're referring to the part of the panel that shows opened windows, it's called the Taskbar applet. Right-click on the panel, select Add Applet to Panel, and add the Taskbar.
Hope that helps.