Some mainboards won't boot without an active graphics chip which in turn won't start without a monitor connected.
This hack works with 3 resistors (75 ohm (50-150 is fine), 0.25 watt (0.125-0.500 watt is fine)).
Putt them like this
Resources:
https://www.techpowerup.com/forums/threads/how-to-make-a-dummy-vga-dongle.86507/
http://www.geeks3d.com/20091230/vga-hack-how-to-make-a-vga-dummy-plug/
http://www.electronics-lab.com/how-to-make-a-vga-dummy-plug/