I have had similar problems when it wasn't even connected up to my tv, and just using hdmi through a monitor... Happened first time I connected it up to the monitor (first time I powered the box on). Thought the box was dodgy, until I restarted and it came up
Also had it when connected through my hdmi amp... Just turning on, and nothing outputting on tv... Now, I just power off the twin, and on again, and it works fine...