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I can't login now so this is from memory. This afternoon I updated all pending packages, of which I believe one was Xorg. After restarting, the fedora icon appears(indicating the GUI is operating) and slowly fills while the system boots. A few seconds after the icon is completely filled, the GUI crashes and the system halts. At this point the only keyboard combination that appears to do anything is Ctrl-Alt-Delete. This restarts the system. Ctrl-Alt-Backspace does not appear to have any effect.
This wasn't an X issue. After looking at the yum logs, removing then updating PackageKit uninstalled some packages but did not reinstall them. Adding them back in manually allowed the system to boot normally.