My Vega 64 stopped working after rebuilding my water loop. Turns out, water leaked from the VRM block onto the motherboard, taking out the PCIE x16 slot and the graphics card. The other possibility is that I reinstalled the backplate or leaf spring incorrectly, making contact with the PCB. There is a region on the card that looks charred. I actually took a picture of the card before reinstalling the block and I confirmed the charring was not there.
Does that area smell burned? Because it doesn't look burned to me. It looks like a capacitor may have corroded away due to electrolysis. That could also explain why your slot died (ionized water running into the slot below, causing a short circuit.)