First you need to make sure "widgets" are unlocked (right click the desktop, there's an option)
Then, if you look under that same menu, (by right clicking the desktop) There's a different option to "Add Panel / Default Panel".
If you had made any changes to the panel, you will have to make them again.
(This response assumes an up to date KDE desktop)
Good luck.
p.s. Keep your widgets locked!