When I start up my Mac, I see nothing but a blue screen. I can move the mouse around, but that’s about it. How can I fix this blue screen?
A Mac that shows you a blank blue screen instead of logging you in may be afflicted by a malfunctioning startup item. Most people have a number of programs that start up whenever you log into your computer, and it’s likely that one of these programs is causing issues. The startup item is unable to load, and so it causes the rest of the computer to hang.
In order to get your Mac past this hanging blue screen, you need to boot into safe mode and figure out which startup item is causing problems. The Apple Support article “I see a blank blue screen when I try to start up my computer” tells you exactly how to go about this procedure. Basically, you must remove all your startup items and add them back in one-by-one, rebooting each time until you find the one that causes the hanging blue screen.