iMac 🖥 suddenly turns of in the middle of start up and won’t completely turn on. What can I do?

You can also attempt to reset the NVRAM per the directions in https://support.apple.com/en-us/HT204063#targetText=How%20to%20reset%20NVRAM,Mac%20might%20appear%20to%20restart.

If this does not work, also attempt to restart in Safe Mode per the directions in https://support.apple.com/en-us/HT201262 . NOTE: You may need to hold down the Shift Key up to 10 minutes for the computer to start in Safe Mode.

If the NVRAM reset, Safe Mode restart and the directions to start in the Recovery Mode and run First Aide on the internal drive do not work, then you have exhausted what you can do. At that point the next step is to take the computer to your local Apple Authorized Service Provider.

Try to boot in safe mode by pressing left "Shift" key at start up.

If it does not work then press "Option" key at startup and see if there is your drive available & for how long it will be available. If you mac does not turns off then probably issue is with your drive. To resolve this problem go to in recovery mode by pressing " Command + R" keys at startup and from there select Disk Utility & repair you drive+partition.

Then boot again and check if problem is resolved.