I am having a similar issue to this. This morning I dismantled a late 2015 iMac 3.2GHz that was yesterday working without issue. I did a routine heatsink service and replaced the thermal pads and paste. I also fitted a Crucial P3 NVME blade with a Sintech adaptor. Everything was cleaned with an electric air blower prior to reassembly.
Before attaching the display I always do a routine diagnostic check to see if LED 1 comes on. But I got no sign of life at all. This was all with the same power cable used with the machine yesterday and the same socket. I swapped the cable out for a known good one and used a different power socket - still no LEDs.
None of this makes any sense to me. I have worked on hundreds of iMacs, not done anything differently and in this instance the machine was working perfectly before. It just had a worn hard drive which was the initial reason for working on it.