By the 8 pin connector you are talking about the PSU cable that goes to the GPU card correct?
Sounds like your RX 570 shorted out and is shorting out the PSU which shuts down to prevent damage.
Check your PSU Outputs with the GPU card disconnected and see if the 3.5/5/0/12.0Vdc are within 5% tolerances.
You can run OCCT PSU Stress test to check for abnormal PSU Outputs and Temperatures.
If the PSU checks good then the RX570 is defective. If you install it on another computer with the same results that will verify that you have a shorted GPU card.
My guess the GPU card might have overheated and shorted out.
EDIT: Remove the RX570 and physically check the PCB and fan for any type of damage like discoloration or broken or missing traces or burnt or damaged components.