reason 1）You got a loose power cord;
reason 2) Your SMU needs a reset:
– Turn off the computer by choosing Shut Down from the Apple menu, or by holding the power button until the computer turns off.
– Unplug all cables from the computer, including the power cord.
– Wait 10 seconds.
– Plug in the power cord while simultaneously pressing and holding the power button on the back of the computer.
– Let go of the power button.
– Press the power button once more to start up your iMac.
reason 3) Your PSU is dead.
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