What does NA mean in the upper right corner of the phone?

A black screen appears during the use of the smartphone. It is recommended to try to press and hold the power button 12 seconds to forcibly shut down and restart the phone. If your problem cannot be solved, please take the purchase invoice, warranty card and mobile phone to the nearest customer service center for testing and handling.

If the black screen disappears after restarting, it is recommended to optimize the mobile phone as follows:

1. There is too much cache in the mobile phone system. It is recommended to clear the system cache.

2. Check whether the black screen often appears when using a software. If you have any suggestions, set up-apply uninstall software to see if it can be improved;

3. Back up data (such as address book and multimedia files) and restore factory settings (Settings-System "More"-Restore factory settings, please do not check "Clear completely").