If the Xiaomi Smart Band 9 Active shuts down automatically, it can be due to several reasons:
1. If the band is not paired with a device and remains inactive for 15 minutes, it will automatically power off to conserve battery and protect the internal components. To reactivate the band, simply charge the band and pair it with a device;
2. The band is designed with a safety feature that triggers an automatic shutdown if the temperature exceeds safe operating limits. This helps prevent overheating and potential damage. After the temperature drops, you can restart the band by plugging it into a charger.