Hmm so a rig stopped mining after being 8+ months stable.
Now all of a sudden it has completely lost the ability to detect a GPU via riser.
Plug directly >>> all GPU work
Plug via riser (any slot, any GPU, any riser) >>> no lights, no fan, no nothing. Yes I tried many many risers from spares etc.
I stripped the rig down to 1 GPU with the same results;
Directly >>> fine
Riser >>> nothing at all. Not even a LED!!!
Long shot, but try to set PCIe to gen1 in BIOS if you haven't already. Maybe the slot is getting old and can't sustain the requirements of the higher settings?... It's worth a try. Especially if all GPUs are affected.
If only one gpu's showing that problem, try switching slots and see if the behaviour remains. Or try another mobo.
Good luck with the bitch that is rig troubleshooting...