Comment 4 for bug 1000690

Please run kdebugdialog and tick everything, then reboot, then try to trigger the issue and attach the file .xsession-errors from your home directory to this report.
Also useful logs:
/var/log/kdm.log
/var/logl/Xorg.0.log

If your session remains fully functional after trying to shutdown/reboot/logout you can simply do that, if not you'll have to switch to a virtual terminal ALT-shift-f1 and copy the files somewhere while your session is stuck trying to shutdown. Point being that the logs get overwritten once you start a new session.

Also a note on comment #3: quite literally you cannot/should not expect halt to do anything more than halt the system (i.e. stop the CPU from doing anything). What you probably want to use is shutdown, which brings down the OS and then stops power supply. for convenience/safety reasons halt also brings down the OS first but it will not actually stop the power supply. halt very much directly relates to the fact that most CPUs have a dedicated instruction to stop them from doing anything, that instruction is mostly called halt (or a shorter version of it like 'hlt'), and that is actually what is called at the end of the program called 'halt'. Strictly speaking not every computer can actually shutdown and/or reboot as those features depend on additional low-level magic such as ACPI to allow power management control and consequently allow software to tell the computer to shut down power supply or shut it down and then fire it up again. With that in mind you almost never want to use halt but shutdown (even though just about all linux/bsd/whatever versions will have halt either do exactly what shutdown does or what you experience on Ubuntu - a OS shutdown combined with halting the CPU).