If your Mac displays a blue screen at startup
If you see a blank blue screen or a blue screen with a colored pinwheel when you try to start your Mac, but starting up doesn’t progress, you may have a problem with damaged or incompatible startup items or software.
Note: To print these instructions, press Command-P.
Restart your Mac in safe mode. If your Mac successfully starts up in safe mode, choose Apple menu > Restart to see whether the Mac will restart normally.
If your user name and login password have been accepted in the login window but starting up doesn’t progress, check your login items. They may be incompatible.
Use the Disk Utility app on your Mac to repair your startup disk.
Back up your disk, then reinstall macOS.
If you’re starting up from a DVD, it must be a disc that came with your Mac. Discs from other Mac models may not work with your Mac.
You can also use macOS Recovery, which helps you restore from a Time Machine backup, reinstall macOS, get help online, repair or erase a hard disk, and more. See the Apple Support article About macOS Recovery.