I have read on more that a website that a conjunctive adverb is preceded by a semicolon. As far as I remember, however, I have seen more often than not a period used instead of the semicolon. Even on the same Web page below a period is used when a semicolon, they claim, is called for,
Notice, however, that you need a semicolon, not a comma, to connect the two clauses:
Main Clause + ; + Conjunctive Adverb + , + Main Clause.
... The cat ate a bowlful of tuna; then, to the squirrels' delight, the fat feline fell asleep in the rocking chair.
On the same page you find an example at odds with their advice or the rule they set.
At 10 a.m., Paul was supposed to be taking his biology midterm. Instead, he was flirting with the pretty waitress at the coffee house.