Had same with my Ultimo4K, happened a few times, then would be fine again for a while.
Turned out the on/off switch is faulty, I found by switching it off, then firmly on again a few times, the box would boot, so must be a dodgy contact inside the switch itself.
I did remove the motherboard and try re soldering the switch incase it was a dry joint, but that made no difference.
Once I found that it would boot and remain booted I have never touched the switch again, I just pull the power cable out and reinsert it now if I need to and it's been working 100% for last 18 months like that.