Did you use Lucid Virtue / Quicksync / multiple monitors (iGPU and 660) before? If you have defaulted your bios it should be defaulting to the 1070 for post.
If you did, disable multimonitor support and the likes related to the iGPU and have a cable only connected to the 1070.
as a last resort disable iGPU in bios. However if you still get a blank screen you will need to remove the motherboard battery to reset the bios to use the iGPU.
EDIT: If you can attatch a second monitor it may be worth while you are in windows on the iGPU, checking to see how many displays windows recognises with a display plugged into the iGPU and a display plugged into the 1070. Before disabling the above obviously.