I had the same problem yesterday (15 Dec 2016).
App Store prompted me to download 3 things: iTunes version 12.5.4, a security update (I think) and Safari update (I think/perhaps). The reason why I'm not sure about the latter two is that although I downloaded them and installed them (and my machine restarted twice), for now the only "Updates installed in the Last 30 Days" according to app Store is the iTunes one (but also says: "No Updates Available")...
Got stuck on load screen after reboot too, then aborted and rebooted twice, which seemed to do the trick, but, as said, the other two updates don't show in "Updates installed in the Last 30 Days", AND... since aforementioned update process Safari doesn't load any more. It's version 10.0.2, modified 25 Oct 2016, but not sure if that's properly installed as according to TM my previous version was 10.0.1, modified 25 Oct 2016 as well!
How can I resolve this?
Thanks a lot in advance for any useful tips!