Apple has released a 10.10.3 supplemental update for OS X Yosemite. The update addresses a video driver issue affecting some Macs during starting when running some apps that capture video, according to the description.
OS X Yosemite 10.10.3 Supplemental Update
The OS X Yosemite 10.10.3 Supplemental Update fixes a video driver issue that may prevent your Mac from starting up when running certain apps that capture video.
You can download the update here, or go to the Mac App Store on your Mac and click the ‘Updates’ tab.
Apple released OS X Yosemite 10.10.3 just over a week ago, which included the new Photos app.