Standard trouble shooting would say take the motherboard out, put it on its box, plug in everything and see if it works. If it does then you were shorting something out in the case.
If it doesn't, recheck the power plugs. The 24pin and 8pin for sure. For this test you only need to have the CPU/RAM/GPU to see if it boots to BIOS.
If everything checks out and still doesn't boot up or the fans don't spin then normally that would indicate a CPU problem. If it starts up but gives you Qcode errors and doesn't boot into BIOS that usually indicates a RAM issue. At this point try the ram two sticks at a time (see manual for where to plug them in).
Good luck.
FYI I have had this happen many times and every time it was the 8pin that I didn't have plugged in correctly.
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