Chosen Solution
My church’s iMac starts to boot and then fails! The Apple symbol and the progression bar displays and the bar gets to about 1/3 the distance and the system shuts down!
Try restarting your system and press and hold the Command (⌘) and R Keys to get into the MacOS Recovery. Then run Disk Utilities on your drive. Here’s a bit more: How to reinstall macOS from macOS Recovery. You don’t need to install the OS just get the system running within the recovery partition. Reference: Mac startup key combinations
