I'm having the same problem... The new software 10.9.5 apparently downloaded correctly, but it's completely stalled at "restarting". There are two indicators, the "progress bar", the one which have those blue and wire strips in a diagonal pattern is stopped, it do not move for about half an hour, and below there is a "progress circle" I don't know how do you name it in English, but it's the one that appears on your iPhone when it's downloading smt (I'll address some photos), and that progress rolling circle is moving!. What am I supposed to do? I've been waiting for a long time and I'm starting to notice that the MacBook air is heating itself so... Please help!
(The upload photo tool appears to be broken...)