Why does "shutdown" option only log me out?

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http://askubuntu.com – When I need to reboot (sadly, the days of 300+ days of uptime appear to be long gone) while logged into my account, the Shut Down option only logs me out - it does not shutdown the system. When at the login screen, selecting the shutdown or restart option does nothing - I get a dialog box, but regardless of which option I select, nothing happens. Interestingly enough, when I first boot up, selecting shutdown from the login screen works, as long as I haven't logged in yet. Is this a known problem? Any way to figure out how to fix it? (HowTos)