I think you may be using some words differently than I do, so let me ask you a couple of quick questions:
By "killed off the desktop" do you mean that you have:
a) No graphical interface at all, just a big black screen with a prompt
b) A graphical interface with a background and a cursor that's responsive to your mouse, but nothing happens when you try to do stuff with the mouse
c) A responsive graphical interface with other things that come with a standard KDE desktop, such as a panel at the bottom, but you've just lost a "sidebar."
I don't know much about scientific Linux, but if it comes with a standard KDE desktop consider the following:
1) The word I think you might have more success with is "panel" rather than "sidebar" when it comes to web searching. I could be wrong, but that's what comes to mind for me.
2) Have you tried clicking on the right mouse button? If the issue is just that you have removed some panels and widgets, that should bring up a little box with the option "Add panel" That will allow you to put a new panel on the desktop, though of course, you will also have to populate it yourself.
If you give us a better handle on your problem, there are a lot of things to try. If *nothing else* works, or if you don't particularly care about what's in your home directory, then...
3) If you have completely destroyed your desktop, then a last-resort kind of thing might simply be to delete all the KDE-relevant stuff in your home directory, log out, and then log back in. That will *usually* repopulate everything with a default configuration, but if it's just that you have screwed up some config files, it allows you start over. Of course, it will also destroy all the customization you have done. The directory you can delete is ./.kde . Note that this can be a bit dangerous.
4) Finally, if you have admin privileges, you can *truly* nuke everything and start over by doing the following:
a) move your home directory and back it up, e.g. "mv /home/myname /home/myname.old" (and back it up also).
b) delete your username, e.g. "userdel -r myname"
c) add your username, e.g. "adduser -m myname"
d) copy important stuff from /home/myname.old back into /home/myname
e) delete /home/myname.old
You will need to reset the password, etc. on your "new" acct.