Have you switched it to use onboard graphics as primary display device in the bios?
Mobo: ASRock H81 PRO BTC R2.0 S1150 Intel H81
GPU's: 6x Sapphire RX570 Nitro+ 8GB
I never had any problems booting the system before as I was setting the rig up whilst the screen was always plugged in and now when everything is set up I must plug the monitor in to boot the system. Never ever had this kind of problem with any other computer systems.
Mainboard Bios -> advanced settings -> disable render stand by.
I'm not sure which suggestion helped more as when I entered BIOS I have change switched on together. Now it boots perfectly from the first time. Thank you!