The PCIe slot is likely dead, do not plug in anything to that PCIe slot. It is possible that the motherboard also died with it, but there is very little chance of this damaging the GPU - the USB cable that connects this pciex1 inset to the riser isnt designed to carry power, and can carry very little.
Disconnect all GPU's, see if the the computer is still booting. If it does, next try checking the GPU's individually. Do NOT even try to plug anything into the slot you burned, try the rest of the slots.
This is 2017 all over again
this was a pretty common mistake for people to make back then.