Today I did some updates, installed programs and some theme customization for gtk apps. When I restarted I can see only a black screen only mouse pointer visible and a notification from checkgmail that new email has come (on top of this black screen). The problem is that I have auto-login for this user and when I start PC I can see this Kubuntu wheel at first and then balck screen so I can't choose to log in to another (administrative) account.
When I press Ctrl+Alt+F2 I can do some terminal work though. Maybe I can save this installation somehow?
I tried:
$ kquitapp plasma-desktop
but I get: "kquitapp (2915) Cannot find application plasma-desktop by using servis org.kde.plasma-desktop and path /mainapplication."
when I type: plasma-desktop I get
QDbusConnection: session D-bus conncetion created before QcoreApplication. Application may misbehave.
plasma-desktop (2914) kuniqueapplication:cannot find d-bus session server:
"Unable to autolauch a dbus-deamon without a $display for x11"
plasma-desktop (2913) kuniqueapplication: pipe closed unexpectedly.

I installed Xfce and gdm and I logged in to xfce without problems and I can access Moun but how to recover KDE?

Muon doesn't show any broken packages.
Edit 1:
I was able to log in to my administrative account to KDE, so the conclusion is that there are some settings in this broken account. but maybe there is a chance to recover not loosing all plasma and KDE settings? Perhaps this has something to do with x server and dbus?

First we need to determine which process has failed. Would it be possible for you to try to run "plasma-desktop" by hand under the broken account from a shell terminal? (such as Konsole)