I was on CentOS with KDE, and I accidentally pressed Ctrl+Alt+Esc, and then clicked the desktop, and the taskbar. Only the mouse pointer worked now. My short research showed that this is xkill, and it kills the process I've clicked on. There are some instructions on the internet, which would help me when either the taskbar, or the desktop is killed, but never both.
What makes everything even more difficult, is that it was at the student's computer centre of my university. I tried to power-off the machine, and turning it on again, but it just connects to the central server, sees that my old session hasn't been terminated, and asks me for the password. There were literally no other options availabe. When I enter it, there's again the black screen, with only the mouse cursor. Logging in from another machine, everything worked fine. The computer is reset tonight, and everything should work fine tomorrow.
Still, I really want to know how you would escape from such a situation. Desktop and taskbar are gone, Ctrl+Alt+F* doesn't work, and hard restarting only asks for your account password again, leaving you the session with the black screen.