Chosen Solution
I’ve had this iMac since 2017 and I’ve never had a problem with it. A few days ago I plugged my fitbit charger into the USB port and the entire computer died. I tried all the the simple fixes I could find (reset SMC chip/taking out power cable/trying a new power cable etc) and nothing worked. After running out of options, I removed the screen and checked the diagnostic lights. Only the first LED lit up when trying to power it on. So I replaced the power supply hoping that would fix it, but I still have the exact same problem. Only the first diagnostic LED lights up. I’m hoping some people smarter than me can give me some suggestions on what might be wrong. Thanks for your time!
OK, so you didn’t force power into your system via the USB port. But the USB port connector could be damaged I would inspect them to see if any of the pins are folded pack hitting the shield area (sorting out). The act of plugging in a cable on its own wouldn’t otherwise hurt your system. So I think if the connectors are OK we need to look at other directions. Clearly you have a sort condition somewhere within the logic board likely a blown capacitor. Carefully inspect the board using a magnifier looking for a blistered component or staining on both sides of the logic board.
