When a Mac laptop starts up and present a grey screen with a folder which has a question mark on it, what does it mean?

When a Mac laptop starts up and present a grey screen with a folder which has a question mark on it, what does it mean?

• The computer cannot locate the OS files
• The hard drive is full
• Siri is active and waiting for a question
• There is a problem with onboard RAM 

EXPLANATION

 A gray screen, as strange as that may sound. The “gray screen” problem can also manifest itself as a black screen; actually, a screen so dark you may mistake the display as being powered off. This is especially true of Macs with built-in Retina displays, such as the Retina iMac models that don't have a power on indicator.

This startup issue is called the gray screen problem because historically, the display would turn gray during the start-up phase when the problem struck. The more recent Retina Mac models have a black or very dark display instead 

Peripheral Issues

RAM issues or Drive Issues

When the flashing question mark in a folder appears at start up, it indicates the computer is unable to find the  files for the OS.  The boot drive may have changed, or the hard drive may have crashed.  See below:

https://www.ifixit.com/Answers/View/236721/A+flashing+question+mark+in+a+folder

SOURCE

https://support.apple.com/en-ca/HT204323