Selective Mutism

If your child has developed 'selective mutism' you might want to read a little about it. BAsically this is when a child is able to understand and speak fine usually but in specific circumstances does not speak at all. It would also be wise to speak with a professional if your child continues with his or her mutism for longer than a few weeks.

Often this behavior is a result of anxiety. Sometimes it is a behavior which children find works for them to get some need met. For example children might not respond to cause distress in adults and redirect the focus or to gain attention when another sibling usually receives most of the caregivers attention.

Each child and situation is unique and should be assessed if continues for a period of time because the cause can worsen.

https://www.google.com/health/ref/Selective+mutism