Hi there.
This might sound really simple, but if you have a control box that switches between OTA/Cable/Satellite/DVD player..etc, check to be sure the Cable button is selected. Also, if there's more than 1 set of those controls on the box (i.e. front TV, rear TV, outside TV...etc), be sure you're checking the correct set of controls for the particular TV.
I can't tell you how many times I thought my TV wasn't working only to discover the wrong button was selected on the control box for the method I wanted to watch.
Lori-