I have black horizontal lines across my computer screen. What is the cause and how do I correct it?

Are these 2 very thin horizontal lines about 1/3 from the top and 1/3 from the bottom?
This is perfectly normal for many types of CRT monitors. For some explanation see here: Howstuffworks "What causes the faint horizontal lines on my monitor?"

Check out the following link, hope it'll answer your question

Horizontal lines across display - View Single Post

There is black line appeared across my computer screen. What is the reason for that?