If you mean Default Desktop (I don’t have a Standard Desktop option), then right click, Add Widgets, search for the Folder View widget and move it on to the Desktop. If the icons do not appear inside it, right click on it, select Folder Options and click Show Desktop folder.
No don’t know. But you must understand that in KDE there are several different ways to display icons on the desktop. So we don’t know if you were using a Folder layout or a folder widget. Note that a widget is used by default. It is all controlled by right clicking the desktop and selecting Folder Settings . Also you can have multiple desktops with or without icons.or a different set of icons. Unless you change the wallpapers they all look the same. So you could have accidentally changed to a Desktop that had no icons assigned and though the icons were gone but they were still on the origianl desktop. If FF was open then selecting it from the task manager would take you back to the original Desktop and again you see the icons.
Don’t know if that happened because I don’t know how you have your system configured but I’d say it is a good guess