A thin black horizontal line on a computer screen can indicate various issues, often related to the monitor or video card. In the case of Trinitron monitors, such lines may be shadows from stabilizing wires, which can become less noticeable over time. For laptops, like the HP Pavilion zv6000, the line may suggest a defect in the monitor's row drivers or physical damage to the screen, especially if it appears after stress is applied to the frame. Testing with an external monitor can help determine if the issue lies with the screen or the video card; if the external display works fine, the problem is likely with the laptop's screen. Some users have found success in fixing similar issues by disassembling the display and adjusting connectors or the inverter board.