Suppose the players of a team are sad over losing a game:
The team just lost a close game. Hence the sadness.
Is the second sentence a complete sentence?
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As the other answers say, yes, it is a sentence fragment. The full sentence might be something like "Hence, there was sadness." That would make the sentece a complete one, because there is at least one verb ('was,' it is not commonly thought of as a verb) and a noun ('sadness').
If you were to say it, it would be acceptable, because everyone simplifies sentences in speech. But if you were to write it in an essay or such, you would have to add just a few words to make it appropriate.