Was there a Helen of Troy?

Yes. she was a Greek demigoddess and was a daughter of Zeus and Leda. She was wife of King Menelaus. Her siblings were Castor, Polydeuces, and Clytemnestra. Later, Paris, helped by Aphrodite, abducted her to Troy. After that she was Helen of Troy.

Other than the Homeric tale and later tales which may or may not have come from an earlier tradition, there is no evidence that Helen ever existed.