I believe it's called the pager, that is, the part of your "master" panel (mine's at the bottom) where tiny boxes representing your open apps are shown, so you can click on them to bring them topside again. Pager, right? Anyway, I suddenly lost mine and in my frustration accidentally removed the master panel, too. So I rebuilt my master panel, but now I have my virtual desktop icons stretching over all available room and no pager. This all started happening when I tried to add a second (external to my laptop) monitor (probs with bug 230872, "dual monitor settings amnesia"). Also, I've had apps open ... and not be on the desktop, but "behind" somewhere. I've got a Window List icon that sees them, but sometimes they're not on the desktop. In general, this new stuff is a pain along with the gain.

Add a spacer widget or a taskbar widget. They fill out the space that pager is using.

I guess you're looking for the "task manager", try to add it to your panel.

If it doesn't work, it would be helpful if you could take a screenshot and post it here.

The pager is used for selecting virtual desktops, as Hans said you are looking for the task manager.