“If your Mac starts up to a gray screen (with no flashing question mark) or a flashing question mark or flashing globe, give the computer a few more moments to locate its system software. If you normally start your computer from an external drive (Thunderbolt, FireWire or USB) make sure the drive is connected and powered on.
If your computer still does not start up, try the next set of steps to help your computer find its system software.
Turn off your Mac by pressing and holding its power button for a few seconds.
Press the power button once to turn your Mac back on. Then, hold down the Option key on your keyboard until the Startup Manager appears.
Select your startup disk from the list of drives that appear on screen.
If your computer finishes starting up normally, choose System Preferences from the Apple menu. Then, click the Startup Disk icon in the System Preferences window.
Select your normal startup volume (such as Macintosh HD) from the list of drives that appear in the Startup Disk window.” check to make sure that you seated all your cables correctly, specially the HDD
