iMac will not boot up after shut down and unplugging

Hey punkandchicken,

That was a really good step to try, resetting the SMC. Have you tried holding down the power button for no less than 10 seconds? That's another good step you can give a try.

• If your Mac doesn't turn on - Apple Support

If the iMac still doesn't power on, you will want to contact Apple Support by going to Get Support. 

Good luck!

Have you previously unplugged and plugged it back in and successfully booted? If so it's very well be a failing/failed power supply. I had the same issue with a 2015 iMac a few years ago.

If following Dorain1's recommendations you're still unable to boot definitely contact Apple support for a free diagnostic test of your Mac. Note: Apple will want to erase the drive and install a new, pristine system to verify it's hardware related and not software related. So make sure you have a full and current backup of your boot drive before taking it into Apple.

Did you plug it back into the power outlet? You didn't say one way or the other.