Recently, I lost one of my monitors. Notoriously, this messes up KDE.
My old panel (aka taskbar) is now invisible, lost in the unmapped reaches of the old coordinate grid I had when I had more monitors, and, although I can add new panels, the old one is still there, capturing my Super (aka Meta aka Windows) key, so I can't raise the Launcher (aka Start Menu) for any new panels. Widgets are in a similar situation, impossible to remove conventionally given that they're invisible and eternally running the background, lost in off-screen purgatory.
I've seen others struggle with this, and I heard of some magical conf file somewhere, full of inscrutable text, that encodes the kinds and positions of your whole KDE desktop (the panels, the widgets, etc.). Wiping out this file might reset my KDE desktop to "factory settings". But where might that be? Or, if that's wrong and there's a better way, what might that be?