I haven't seen anybody suggest trying the cards on another machine (apologies if I missed it), so I'm going to ask if you have the capability to do this? Try plugging the cards, one by one, into another board. See if they are able to work in another system, because it's entirely possible that something goofy happened and anything from the GPUs to the Mobo could be malfunctioning. You need to isolate the issue, do you have any other GPUs you know to be working that you can test within the Gigabyte Z170X-UD3 motherboard?

Are you able to get any video from the cards? GPUs come with a low resolution output that is standard and native. It'll show your BIOS, safe-mode, all that good stuff.