Why is my TV monitor showing horizontal black lines

Usually if there's black lines on the screen it's either an issue with whatever you have plugged in, or an issue with the LCD itself. Easiest thing to check would be to plug another thing in to see if the lines are still there, and if they aren't then it's an issue with whatever you had plugged in.

If the lines are still there, then you could try to powercycle your tv on the off chance it's a software issue. Just unplug your tv from power for about 30 seconds to a minute (it'll be a three pronged plug) and plug it back in. If the lines are gone, then it should be okay. If they show up after a little while of using the tv, then it might be a heating issue.

If after a power cycle the lines are still there, you could double-check to see if it's on a different setting (Some tvs have a game mode that can be a little funky with some devices). Change the mode around (Or you could also factory reset the tv as a last-ditch issue) and see if that fixes it.

If all of this fails, it's most likely an issue with the tv. You might be able tog et a replacement from the company, but either way it'll need to be replaced unless you want to do some soldering.