Chosen Solution
When I turn on my iMac you can hear the chime and the Apple logo and the loading bar starts to load but about halfway through it loading up it just suddenly shuts off.
Let’s give this a try: Reboot your system an press the Shift (⇧) key to enter into Safe Mode. Are you able to get to your system to fully boot up? If not lets try one more test here we want to enter the system into Target Disk Mode pressing the T key did that work? Reference: Mac startup key combinations
