A system or application BUG may cause a black screen and restart the phone. After the restart, upgrade the system, restore factory Settings, and delete recently installed applications.
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It may be that the health of the mobile phone battery is too low, resulting in abnormal phenomena such as deadlock and crash of the mobile phone, try to replace the new battery.
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The system may identify screen anomalies (including screen faults, motherboard faults, etc.), and need to contact Apple after-sales service for further maintenance.