Mac System Show Big Gray Screen while Starting?

When you try to boot into your Mac system and start it for an important work, you suddenly comes
across the login window or Finder. But if the startup process doesn't accomplish successfully and you observe unexpected system behavior, you might need to perform 'Erase and Install' to solve it. If a valid backup is not available in such cases, you need to use a professional Mac Data Recovery application.
For an instance, consider that you start your Mac OS X installed system and observe one of the following items displaying on screen:

When the Mac screen comes up some of the big items after start up like

Spinning gear
Apple logo
Folder with a flashing question mark
Spinning globe
A gray screen that does not last.

The above issues might occur if any of the following components is faulty:

Peripheral devices or network cables
PRAM/NVRAM
Startup volume structure
Startup volume permissions
System files

To come up with these issues you may try the following suggestions:

Try disconnecting all the peripheral devices from your machine. Also disconnect Ethernet cables, if any. If the system is able to start, try connecting the disconnected devices one at a time to determine the faulty one.

Try booting the system in Safe Boot

You can try resetting PRAM/NVRAM

Repair the Mac OS X volume using Disk Utility through an installation disc. Also, repair the volume permissions

Make sure that all the cables and power source are perfect and in good condition

If you are using a third-party internal hardware or RAM, you should remove it

Reinstall Mac OS X using 'Archive and Install' method

If you still cannot resolve the existing issue, you should perform 'Erase and Install' to reinstall Mac OS X. Restore the data thus lost from backup.

It is important to take a backup of your data always however If you find any issues while restoring data from backup, you are suggested to immediately use a Data Recovery Mac application to extract it. Mac Data Recovery software are safe data recovery tools designed with high-end algorithms to locate and restore all the lost data from Mac drives.