My iMac won't boot ( internal and external )
Repair or restore a Mac in Apple Configurator 2
If there is no way to get the Mac working again, you will need a second Mac.
The firmware must be restored and the internal flash memory must be deleted if the following situation occurs:

The Mac cannot start from Startup Disk or RecoveryOS.
The firmware repair failed.

Repair or recovery process
The repair or recovery process consists of the following steps:

Start Apple Configurator 2 on the second Mac and connect the Mac computers.
Restart the Mac using a specific key combination.
Use Apple Configurator 2 to do one of the following:
Repair firmware and reinstall the latest version of RecoveryOS
Restore firmware, erase all data, and reinstall the latest version of RecoveryOS and macOS


Step 1: Configure Mac with Apple Configurator 2 and connect the USB-C cable

Make sure of the following:


You have installed the latest version of Apple Configurator 2.
Your Mac is connected to the electrical outlet.
You can connect to the internet.


Connect the USB-C cable (along with any necessary adapters).

Step two: prepare your Mac

Connect a monitor so you can see when the process is complete.
Unplug your Mac  from its electrical outlet for at least 10 seconds.
Press and hold the power button.
Reconnect to the power supply while continuing to hold the on / off button.
Release the power button.

Note: You will not see any Mac activity on the screen during this time
Step 2: prepare your Apple notebook computer

Press the power button.
Hold down your power button while simultaneously pressing the following three buttons for about 10 seconds:

the right shift key
the left option key
the left Control key

Immediately release the three buttons after 10 seconds, but keep holding the power button until it appears in Apple Configurator 2.

Note: There is no activity on the Apple notebook computer on the screen during this time.
Step 3: Repair the firmware and install the latest version of the RecoveryOS
In the Apple Configurator 2 device window, select the Mac whose chip firmware is to be repaired and whose RecoveryOS is to be updated to the latest version, then select
Actions &gt; Advanced &gt; Repair device and click on Repair.
Note: Hold down the &quot;Control&quot; key and click on the selected device, choose &quot;Advanced&quot; &gt; &quot;Repair Device&quot; and click &quot;Repair&quot;