Given the efficiency and convenience of sleep mode on Mac systems, it is often a rarity that we end up restarting our Macs. When we do, we expect it to boot to normal operation; however, sometimes a snafu may result in your Mac booting to a gray screen, and no further. Often such behavior happens because of a specific problem, such as a power outage, or the installation of a new software package, but regardless, if it happens there are essentially only a few things that you can do.
Testing your Mac
1. Safe Mode
First, reboot your Mac and hold the Shift key down immediately when you hear the boot chimes, in order to load into Safe Mode. This will bypass all but essential system software, so if a problem is with a third-party system extension, then this will suggest you need to tackle your software and add-ons to fix the problem.
If Safe Mode does not allow your system to boot, then the problem is deeper in the OS configuration and drive setup.
2. Recovery Mode
Next, try booting your Mac to Recovery Mode by holding Command-R immediately when you hear the boot chimes at startup. This will load your system’s built-in recovery disk that has a minimal version of OS X installed. This drive’s contents cannot be tampered with under normal circumstances, so it should boot just fine if your Mac’s hardware is working properly.
If for some reason standard Recovery Mode is not working, then next try restarting your mac and forcing it to load Internet Recovery by including the Option key along with Command-R. Provided you have an active Internet connection, your system should download the ~650MB recovery mode disk image from Apple and then boot to it.
3. Target Disk Mode
Most Macs support a boot option called Target Disk mode, that allows your hard drive to be accessed as an external drive via Firewire or Thunderbolt. This mode only loads limited hardware components, so it is not a good way to test your Mac’s hardware; however, it can be used to test your hard drive. By attaching your system to another Mac, the drive should mount and become available, where you can use Disk Utility to verify and repair the system. You can also manually back up any files you may need.
If you cannot mount your drive in Target Disk mode, or find it is slow to access, then this indicates a problem with your drive and you might need to have your Mac serviced.
4. Hardware Tests
If you cannot get your Mac to load in Recovery Mode, then it is time to test its hardware for any detectable errors. To do this, reboot your system and hold the “D” down on your keyboard when you hear the boot chimes. To boot to the Internet-based hardware tests, do the same but hold Option-D instead. This routine will load and run Apple’s tests and ensure all hardware on your system is running as expected. If an error occurs, you will see a reference to it output on the screen; however, if not then it will indicate no errors are found.
Recovering your Mac
If any of the above tests fail to work, then you likely have a hardware problem that cannot be overcome on your own, and it is time to schedule a repair for your Mac. However, if you can access these modes without error, then you can perform any of the following approaches for fixing your system:
If Safe Mode works, uninstall recently added software or undo other recent changes that may have contributed to the problem. This can be done using official uninstallers that are distributed with programs, or by contacting developers for uninstall instructions. You can also access reputable troubleshooting resources to determine what exactly might be done for specific software errors.
If Recovery Mode works, then consider reinstalling OS X using the tools presented to you. Granted you can restore from a recent backup, but not knowing the reason for the hangup at boot means any number of recent backups may contain the faulty configurations that are causing the problem. On the other hand, reinstalling OS X should not overwrite your personal files at all, and only replace system software files. Provided your hard drive is in working order, this should get it to boot again.
Wipe and reinstall OS X if a basic reinstall does not work. To do this, first ensure you have a full backup of your system, and then use Disk Utility in Recovery mode to erase your hard drive. You might consider re-partitioning and formatting your drive, but at least erase it in full. Then return to the tools menu and again reinstall OS X from scratch. When done, you can use the OS X Setup Assistant to migrate your data and applications from your backups. This approach will ensure OS X is fully operational, and will copy files from your backups to your Mac in a way that is supported and should ensure the system remains functional.