If you have a Samsung LCD TV and an Apple TV connected to it, maybe the HDMI signal from Apple TV is lost sometimes - in particular when you turn off/on the TV.
I corrected my as follows (thanks to Marco in Apple Discussion Forum) :
You’ll have to access the Service Menu on your TV.
** IMPORTANT - Be careful! This menu has very important parameters which you should not change at all. You’ll could “brick” your TV if you change the wrong parameter. **
Turn off the TV
If you have a 52" Series 6 model (my case), press
‘Mute’ ‘1’ ‘8’ ‘2’ ‘Power’
If you have a Series 5 model, try INFO MENU MUTE POWER.
The TV will start and you will be in the SM (give it a few extra seconds).
Using the arrows/enter buttons, go into ‘Option Table (Service)’
Turn OFF the following parameters :
hotplug and hotplugctr.
Turn off the TV.
It worked for me.