There are two main barriers of communication: unwillingness to communicate and inability to communicate.
If one of the participants is unwilling to communicate, it is obvious that communication will fail to be. Another, less obvious barrier of communication is when a participant is unwilling to understand, or worse yet, unwilling to tolerate emotionally.
Such unwillingness often stems from subconscious fears.
Inability to communicate can be present because of obvious physical reasons (eg. you can't send an email if your internet connection is down). However, inability to communicate can also be present because the speaker does not understand his thoughts sufficiently or otherwise fails to formulate them into words.
Such inability often stems from insufficient understanding of oneself.
There can be plenty of barriers of communication, and in case of a breakdown of communication analyzing the barriers can point to a helpful solution.