This is under the category of, “Things you do rarely, and will forget”. How to fix OSX if it does a “blue screen” hang. This is different than Windows — The Blue screen says nothing, and is during the boot process. Remember, OSX is really a Unix, based off of NeXT’s Mach( which is based off BSD). You can repair it like any Unix/Linux box.
Boot to single user mode by pressing Command-S during the boot process, after the start-up gong.
check the disk.
Mount the main disk and delete/move the old preference files. Move them if you want to later restore them ( in case there was something in them you needed )
to mount the disk, run “mount -uw”.
These files to delete or rename(your choice ) are: /Library/Preferences/com.apple.loginwindow.plist and /Library/Preferences/com.apple.windowserver.plist
Note — Deleting those files will force the Login process in OSX to rebuild them — it treats them missing as a “First run” when you next reboot.
I gleaned this information from Apple support and other docs on the web. Use them at your own risk. Usually, if your Mac got to the point where it blue screened, chances are good that you have a hardware problem, most likely a failing disk. There’s a good chance that any reboot will be the last one…