ok, first of all i just want to say i haven't used kde for quite a while so i don't know the proper names of the things i'm going to mention, but i do know what you've done. You've accidentally removed an applet in your panel. Go down to the bottom right corner where there should be a little symbol, and click it. It'll show a bunch of icons in a slidy bar; these are applications you can add to your panel. The ones that have ticks are the ones already there. You need to add an applet i pretty sure is called "task manager". It'll appear to do nothing until you open a window. If it appears on the wrong spot on your panel, while you have the panel options open, (what you had open when selecting the applet) left click the space where the task manager is and move the cursor over to where you do want it. Once your happy with the way you want your panel, you might want to go up to the little symbol in the top right corner, click it and select lock widgets, to avoid accidentally removing something again. But keep in mind, if you want to add something in the future, you must then unlock it first.
(God so many spelling mistakes i have to fix after typing a message like this, lol)