RIG won't boot/start

Hello,

I upgraded from w8.1 to w10 since the os didn't recognize one GPU of the six in the rig.

I installed each GPU separately, tested it, and flashed it.
Now, when i connect the 6 at the same time -- rig won't boot: all fans (included the CPU one) turns on, but nothing shows on screen and keyboard and mouse led doesn't turn on. Why is this happening?

PSU: EVGA 1300W
GPU: 4x RX480 8GB MSI + 2x RX480 8GB SAPHIRE NITRO+
RAM: 8GB DDR3
MB: ASROCK H81 PRO BTC
SSD: 120GB KINGSTON
OS: WIN10

Update: Tried connecting the monitor to each GPU, it doesn't boot. (using a hdmi-vga adapter)
Thank you