Silence in a dream suggests that you are getting silence from someone in waking life. Is someone no longer speaking to you? Are you waiting to hear back from someone and they are not responding? Are you not receiving the appreciation or recognition you feel you deserve?
If you cannot speak in a dream then there must be something in waking life that you are not speaking up about or perhaps you feel your thoughts or opinions are falling on deaf ears. Might be time to change your strategy. Sometimes a letter goes a lot farther than a conversation.