KDE Plasma desktop
The KDE panel and the widgets are plasmoids or plasma widgets. More of the KDE plasma: http://userbase.kde.org/Plasma .
How to manage the KDE plasma panel: http://userbase.kde.org/Plasma/Panels .
Probably you have removed the "Task Manager". More : http://userbase.kde.org/Plasma/Tasks
You could add it back (KDE userbase):
To place a widget in the panel first open the Panel Toolbox and from
there open the Widget Explorer (click Add Widgets), locate the widget
you wish to add and drag it to the desired position in the Panel.
You could move plasmoids: Open the Panel Toolbox and move mouse cursor over the widget. The cursor should change to the "compas rose". Move the widget to the right place.
Same from the KDE userbase:
The panel can then be configured by clicking on the toolbox located on
the right for horizontal or bottom for vertical panel. At that point,
the panel will be in configuration mode. Panel configuration may also
be accessed via the context (right click) menu in the panel.
Widgets in the panel can be sorted by dragging them when in
New default panel (KDE userbase):
To add a new panel either click on the desktop toolbox button and
select Add Panel or open the context (right click) menu on the desktop
and select Add Panel from the menu.
If more than one kind of Panel is installed, you will be able to
select between them to choose what sort of Panel you would prefer. By
default, two Panel types are available: Default Panel, which creates a
Panel containing all of the default widgets (launcher, pager, tasks,
system tray, clock, etc.), and Blank Panel, which adds an empty Panel
without any pre-configured widgets in it.