I have the same motherboard Asus Maximus VIII Formula, same cpu i7-6700k, purchased 6/2016. A month ago, I powered down (even the power strip) at the start of a t-storm. Next morning, nothing, no lights, fans, or P/S fans. P/S tests ok with 4-5 pins shorted. Pulled GPU (AMD RX-480), re-seated ram, all power plugs to m/b, opened M2 cover, shorted cmos battery with screwdriver (can't remove due to armor). Pressed CLR CMOS button on rear, plugged it in and it worked, Went to UEFI bios first to set time.
Next day same thing happens afterI power all the way off, so I know it's cmos battery. Now it fixes by CLR CMOS, w/o re-seating. Problem is, I don't want to remove cpu cooler to get armor off to remove battery (poorly engineered, ASUS, you could have had a small door). So I used my Dad's M3 metal shears to cut the armor plastic in 3 places to allow the CR2032 battery to be removed. It's the triangle shape plastic part between the 5th PCIe X1 card and the battery and the M2 card. R&R battery and now works fine. I was worried because the last m/b from ASUS died on me after 5 years (Asus P8P67pro with i5-2700k). Hope this helps.