List of troubleshooting steps to fix Mac grey screen of death
Disconnecting and Testing External Devices: If you have any external devices connected to your Mac system, such as USB drive, external hard disk, printer, scanner, disconnect them all and shut down your system safely. To find the device that's causing these issues, try the trial and error method by re-connecting the device one at a time and start the system. If the system boot's up properly then continue reconnecting other devices until you are stuck on the grey screen and you need to be sure that you are testing devices one at a time so that you can easily locate the device which is causing your system to get stuck on the grey screen. After performing this trial and error method, if your system still shows a grey screen, then you can try by performing a "safe boot" to see if that solves the problem.
Start system using “Safe Boot” option: Shut down the system or hold the power button until the system shuts down. Now, restart again by holding down the shift key, this makes your system run in safe mode i.e. safe boot which performs various system checks and repairs on the system, including disable all start-up and log-in application. If the system starts up properly, immediately shut down again, and start restarting. If the system again gets stuck on the grey screen, then try the next step.
Resetting PRAM and NVRAM: In order to reset the PRAM and NRAM on your Mac, shut down the system and restart by holding "command-option-P-R" keys and make sure that you have pressed these keys before grey screen appears. Keep holding the keys until the system restarts again and hearing the startup sound twice. Release the combination keys after hearing the second startup beep sound. If your Mac system is still stuck on the grey screen then follow the next step.
Repairing OS X using Mac Startup Disk: After trying the above-mentioned steps, startup using Mac OS X disk and running disk utility program from the menu at the top of the screen. Startup the system by inserting the disk into the optical drive and keep pressing C key until the system starts. The disk utility application can perform repair operations on the hard drive and resolve issues of the grey screen of death. In case this issue is not resolved then contact authorized Apple service provider to fix this issue.
This grey screen issue on the Mac system may also lead to data loss. In case if you have stored important data and you wish to restore, then you can make use of Remo Recover Mac software that can efficiently perform data recovery from Mac even after encountering the grey screen of death.