A black line on mobile screen is a common issue that can arise due to various reasons, including hardware damage, screen defects, or software glitches. It may appear vertically or horizontally and can interfere with the user experience by obstructing the view or affecting touch sensitivity. In some cases, restarting the device or performing a software update may resolve the issue if it's caused by a temporary glitch or app conflict. If the problem persists, you can try a factory reset after backing up your data. However, if the black line on mobile screen remains after these steps, it's likely a sign of physical damage, such as a faulty display panel or a loose internal connection. In such cases, professional repair or screen replacement may be required. Ignoring the issue can lead to further screen damage, so it’s best to consult a certified technician to diagnose and fix the problem accurately.