A：1. Modern mobile phones have proximity sensors. When the proximity sensor above the screen is blocked (e.g., by placing it to your ear or covering the top of the screen with your hand), the screen will turn off. This is normal.
2. This is a power-saving feature. The device has a built-in light sensor. When you bring the device close to your head during a call, the light sensor detects the dimming light and turns the screen off to save power. When you move the device away from your head, the screen returns to normal brightness.
3. If the screen remains black even when there is no obstruction above it (e.g., when the phone is placed on a table), please check the sensor area for dirt or grime. If present, please clean it. If there is no dirt, try restoring the system to factory settings. If the problem persists, you will need to contact an authorized service center for repair.