Stuck on boot screen progress bar after updating to 10.10.5

I just downloaded the 10.10.5 update from the App Store. The Mac automatically restarted. Then it completed some kind of progress screen. Now it's stuck on another progress screen (just like the image below, but with an empty progress bar). No progress in over an hour.

Potentially helpful information:

- it was connected to a familiar password protected wifi network when it restarted. I haven't moved it

- there was plenty of disk space when I shut it down

- I'm connected to power

I've found a few tips on other sites, but I thought I'd ask here since I tend to get better results here. I imagine there is an article or other discussion on this (even though I didn't find one with a few searches).

Thanks!