First try this:

1. Clean the sensor with rubbing alcohol and a mircofiber cloth.
2. Make sure there is nothing interfering with the sensor.

If that doesn't work, reset your unit: Press the power button on the bottom or the back of the unit (depending on the model) to turn it off. Leave it off for 20 seconds, and then press the power button again to turn the unit on.

This might happen after user tries to flip the mirror manually or object is in the way while mirror is auto-flipping. The mirror auto-protects the gear. To reset, simply double tap the touch control bar. You should be able to hear a click sound or see the mirror moves slightly, double tap on the touch control again for another 2 - 3 times and mirror should be able to flip normally.