If 20s doesn't cut it, hold the power button until the frontlight turns off, as that's a good tell that it actually tripped the hard poweroff (assuming it wasn't off to begin with, in which case, just hold it for A Good Long While (TM)).
You won't actually see the power LED do anything until the bootup sequence (e.g., the "press the power button for a bit after waiting a few seconds" part of @DNSB's instructions).
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