The word then is an adverb when it means "next" or "at that time." When it is an adverb, it does not take punctuation.
...We were then seen by a different doctor.
...I then noticed the spot it had left.
When then is pulled out to the front of the sentence and is being used to connect two sentences, it is called a "conjunctive adverb." It takes a period or semicolon in front and no comma after because it is one syllable.
...My husband was trying to talk to her; then he called the doctor.
...I was driving southbound on Montana; then I turned left.
Sometimes the word then has no meaning at all. It is used as a throwaway word. In this case, it takes commas.
...I am following his directions, then.
...Are you saying, then, that he is not a part of this?
Happy punctuating!
Margie