Horizontal lines on a laptop screen can be caused by various factors, including hardware issues, outdated graphics drivers, incorrect screen resolution, and improper video cable connection. Here are some possible solutions to fix horizontal lines on a laptop screen:

1. Reconnect the video cable: Incorrect video cable connection can cause horizontal lines on the laptop screen. Try reconnecting the video cable to the monitor to resolve the issue.

2. Change screen resolution: Wrong screen resolution can also cause horizontal lines on the laptop screen. Adjust the screen resolution to troubleshoot the error.

3. Update graphics driver: Outdated graphics drivers can result in various errors, including horizontal lines on the laptop screen. Update the graphics driver to see if it fixes the problem.

4. Perform a clean boot: If the above methods do not work, perform a clean boot. If the monitor works properly in clean boot, it indicates that some third-party software is causing the issue.

5. Check for hardware issues: If the above methods do not work, it may be a hardware issue. A faulty video card, malfunctioning LCD panel, or a cracked display can cause horizontal lines on the laptop screen. Check for hardware issues and replace the faulty component if necessary.

It is important to use the laptop on a hard surface to avoid overheating, which can cause screen flickering and lines. Additionally, keep the laptop and monitor clean to avoid dust and debris buildup, which can also cause display issues.