Browse the forum for many posts about this. Search on HDMI or CEC.
There seems to be two scenarios.
The most common seems to be caused by holding down the POWER ON/OFF button on the remote control too long. The remote control sends repeated ON/OFF/ON/OFF commands which confuses the HDMI link - on my Panasonic it is called Viera Link. For example, see Unable to turn FVP 4000T on - intermittently! and other "black screen" posts.
The second scenario is to do with the box just having started to do a recording. Alex4000 did some excellent analysis at Failure to turn on when timer recording
Humax technical gurus are apparently aware of both issues and I would expect both to be resolved soon with a software update.