Different traditions exist regarding what happened to Helen after the Trojan War. If we read Homer's Odyssey, in particular Book 4, we find that Helen has returned to Sparta with her first husband, Menelaus. At Odyssey 4.351ff, Menelaus recalls how, after leaving Troy, he was delayed in Egypt for some 20 days (one would assume that Helen was with him although Homer does not mention this).
Euripides' Helen expands upon the title character's adventures in Egypt. In this play, we hear that the real Helen never went to Troy, but rather a phantom Helen. The real Helen was carried off by the gods to Egypt, where she stayed in the kingdom of Theoclymenus, who wanted to marry her. Helen resisted this Egyptian king for many years until she was finally rescued by Menelaus on his way back from Troy.
According to Pausanias (3.19.9), some sources say that Helen and Menelaus were buried in a place called Therapne, while others say that after Menelaus' death, Helen went to the island of Rhodes, where the queen of the island, Polyxo, had some of her maidservants attack Helen and hang her from a tree (3.19.10).
Pausanias also knows of a tradition which finds Helen married to Achilles on a place called White Island (3.19.13).