What is likely cause of no power to iMac.

This is a symptom of two things: one, bad capacitors or power supply, and two, believe it or not, the power cable not being inserted tightly enough! #2 is easy to check right away.

If it's not the cable, open it again and look at the capacitors. If any are swollen or leaking, that's your problem. Also shine a bright flashlight into the power supply; you can see a couple of caps in there, and I have actually found some bad caps that way.

To reset the power system on this iMac you unplug it for a few minutes (unplug everything), then reconnect the power cable while holding the power button down. It's a little awkward as you have to kind of embrace the computer to do this. See if that helps.

You can also zap the PRAM and reset the NVRAM, though those aren't likely to help.