it seems odd that it just dissapeared. im assuming you can run applications from your kde desktop ok.
try checking the panels section in kctonrol (at the kde desktop, press alt+F2 to bring up the 'run command' dialogue, then run 'kcontrol') and check it has 'main panel' selectable in its 'settings for' pop-down. whilst your there, check its hiding settings too, just to make sure its not automagically hiding it for you. and yes, i know these are the 'obvious' things, but its always worth checking them, just to be sure.
iirc, the kde desktop is handled by the application 'kdesktop' and the panels are handled by 'kicker', although dont quote that as gospel, its been some times since i poked around in kde's internals.