The good news is that selective mutism is very treatable with the right care. Kids with SM respond best to behavioral therapy that is focused on helping them learn to speak in new settings, during new activities and with new people.
If you are concerned that your child might have SM, you should get a comprehensive evaluation that establishes a diagnosis. The evaluation should specifically examine the circumstances in which your child is verbal and non-verbal, and if she might have any co-occurring conditions (like other anxiety disorders). An evaluation will also rule out other diagnoses, such as a communication or language disorder, which could also be causing restricted speech.