Hello,
I run Linux Mint 17.1 with KDE 4.14.2 as installed with the Mint 17.1. Unfortunately since some update two days ago my desktop is broken. When I boot the system, I get the "cogwheel" alright, but then - instead of the normal flash picture of Linux Mint + KDE (it builds itself up within a couple of seconds) my various autostarted applications (eg. LibreOffice, Firefox, Thunderbird, Kate,...) start but the control bar (in my case at the bottom of the screen) does not appear. Neither can I switch virtual desktops. I guess this has something to do with kdm. In any case I am at a loss to know what to do - I certainly don't want to go through reinstalling all of Mint. Reinstalling KDE 4.14.2 didn't help either!!
Any ideas (in spite of this rather vague description)?

Check whether "plasma-desktop" is running (either press ctrl+alt+del to get a process manager or alt+f2 to get a runner where you can start "konsole" and "ps ax | grep plasma" there)

If not, try to run it (from konsole) and check whether that works - or you get error messages and maybe a crash dialog.

Hello and thanks for helping!
As I mentioned before, I reinstalled kde-full - didn't help! I also did an update/upgrade through aptitude (didn't install anything in any way connected to the problem and - of course - didn't help either!).
Looking at syslog immediately after booting to a command-line window, I see the following message:
Cannot load /usr/share/kde4/apps/kdm/faces/.default.face: Datei oder Verzeichnis nicht gefunden (file or directory not found).
When entering "plasma-desktop" from the command-line, the result is:
QDBusConnection: session D-Bus connection created before QCoreApplication. Application may misbehave. (I have seen THIS message before in a completely functioning "normal" system!).
plasma-desktop(3070): KUniqueApplication: Cannot find the D-Bus session server: "Unable to autolaunch a dbus-daemon without a $DISPLAY for X11"
plasma-desktop(3069): KUniqueApplication: Pipe closed unexpectedly.
Entering "ps ax | grep plasma" returns:
3119 tty1 S+ 0:00 grep --colour=auto plasma (am I correct in thinking that the reply refers to the "ps ax..." command entered and that the plasma-desktop is actually NOT running?).
Could X11 be the culprit?
Regards from Salzburg
Heinrich

immediately after booting to a command-line window

You're on runlevel 1 or 3 here, what does not fit the description of your original post ("my various autostarted applications [...] start")
You need to have seen kdm or sddm (graphical login) and log into a plasma session.

If you want to operate the GUI session from a different VT (ctrl+alt+f2) you need to export some environment variables, notably "export DISPLAY=:0"

am I correct in thinking that the reply refers to the "ps ax..." command entered and that the plasma-desktop is actually NOT running?

Yes.
However if you just entered runlevels 1 or 3 that's not much of a surprise (no GUI application runs at this point - and they all require an X11 server)