OS X Yosemite: If you see 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.
If you are starting up from a DVD, it must be a disc that came with your Mac or OS X retail package. Discs from other Mac models may not work with your Mac.
Restart your Mac in safe mode. If the Mac successfully starts up in safe mode, choose Apple menu > Restart to see whether the Mac will restart normally.
If you see the login window prompting you for a user name and password, but starting up doesn’t progress, check your login items. They may be incompatible.
Back up your disk, then reinstall OS X.