I should have 4 virtual desktops. But after some playing with the menus,
task bars, panels, etc. There went my 4 virtual desktops.

Not only they are gone, but also I don't know why I cannot use keyboards to
switch between them. The manual of KDE says that I can use "Ctrl + TAB" to
switch. But it did not work.

Previously I have also been used to using "Ctrl+ ARRAW" to switch, that
worked well on GNOME, but not for KDE...

How to get back my 4 virtual desktops?

Thanks a lot,