TV Screen is Dark. No sound or picture

I don't watch tv that much.  Starting about 2 weeks ago whenever I turn on my tv there is no picture or sound.  It seems like it is freezing up.  I have to unplug my box and plug it back in to get xfinity to work.  Xfinity sent me a new box last week and I still have the problem.   If I unplug the hdmi cable when this happens it doesn't fix the problem either.  Any ideas?