Mac Starts Up to a Question Mark service

In order to quickly diagnose and fix the mac question mark folder issues for your MacBook, iMac, Mac Mini and Mac Pro, you need to understand how an Apple Mac loads the macOS from the hard drive.
When you turn on the Mac, the CPU will perform a task named “Power On Self Test”. Your Mac will chime after passing the test, then search for a “loader" from the available bootable-devices such as hard drives, optical drives or USB drives. If no bootable-drive is found, or the bootable-drive found does not have a working loader, your Mac will display a flashing folder with question mark. If a valid loader is found, your Mac will display the Apple logo. The CPU then executes the codes of the loader and loads the rest of Apple’s macOS software into your Mac. 

Depending on what causes the Mac question mark folder issue, the corresponding solution varies from easy to hard. We will discuss the easy fix first and the most difficult one at the end. Usually, software-related issues are easy to fix. The hardware-related issues most of the time require some equipment and(or) spare parts. At the beginning of each section, we will mention the difficulty level of the fix and equipment used so you can estimate your successful rate based on your skill. Good luck!