With the advent of digital broadcast television the ghosting, snowy interference, wavy horizontal lines and picture degradation that was common has been replaced with a blocky, pixilated picture when reception is poor or subject to environmental interference. In some cases, poor reception is manifested as no picture at all. Horizontal lines on a modern screen are likely to be related to the picture aspect, where the picture supplied by the broadcaster doesn't fit the television's settings.
Move the television away from electrical motors like hair dryers or kitchen food mixers if you experience horizontal lines on the screen. Try turning off electrical appliances near the television one-by-one to see if you can eliminate any interference.
Change the aspect ratio if you see large black lines at the top and bottom of the screen. They are caused by the media being filmed in an aspect greater than that which the television can show. Press the button labelled "Aspect" or "Zoom" and toggle through the options. Options can include "Normal," "Cinema," "Zoom," "Wide" and so on. Try the different settings until the black lines disappear. One drawback with removing the black lines is that some of the picture may become cut off.
Change the settings at the source if the problem persists. Your cable box may well have an aspect setting that you can adjust. Look for a "Settings" button on the remote control. Adjusting the settings will make sure that whether the broadcaster sends a 16:9 aspect, or a 4:3 aspect the image will be correct. If your television is a 16:9 aspect TV, try "Zoom 16:9." The black lines will be gone, and the picture will fill the screen, although parts of the picture at the left and right will be missing.