When a trauma is too great (especially seen in early childhood development), it is common to suffer from Selective Mutism.

If you think about it, it really does make a lot of sense. There are some horrors that words cannot adequately describe. Verbalizing trauma is often difficult when you could not speak during the trauma, especially if you had no concrete understanding of what had taken place. Selective Mutism is often seen in severe anxiety and going over traumatic experiences causes an immeasurable amount of anxiousness, understandably so.

I've suffered from a few brief periods of this myself while in therapy sessions and have a friend who cannot speak unless he's alone with me.