Hello , my name is Jack Perhaps I can help you with this problem but first i would have a few questions;
Was the tv working o.k when you replaced the cable box or did you replace it because of this problem ?
Do you have other equipment like a dvd,bluray,games etc. connected to this tv ?
If so , do they seem to work correctly ?
Just a black screen ?
Can you see the TV's menu or anything appearing on the screen when you press the INPUT button on the TV's remote ?
O.k then , not sure what happened here but if the screen is completely dark and you cannot even see the menu or anything else then it does not look like a cable box problem anymore .
Does the tv respond to anything like on/off ?
If not then although I do not have great hopes in this procedure , here is something for you to try ;
Unplug the TV's power cord and hold the power button directly on the tv ,keep holding it for 20 secs or so then let the tv sit unplugged for a couple of minutes before you try it again.
Let me know how it goes and please take the time to rate my answer if that does the trick , otherwise do not rate at all , just let me know and we`ll see what can be done.
O.k then this is some progress but basically you are back to where you were yesterday.
Can you tell me what type of cables/connection you are using ?
Is it a single HDMI or rca type color coded cables ?