A man from Kovno who emigrated to America in 1939 had planned to send for his wife as soon as was feasible. In 1940 he sent a coded message to her to join him, and she replied via telegram that she was on her way. After that, he never heard from her nor saw her again. In 1962, Rav Moshe ruled that it is a fair assumption that she was murdered along with so many others in the Holocaust, and that since the only reason that he could not remarry is the cherem of Rabbeinu Gershom that forbids polygamy (which thus ranks below a Rabbinic prohibition), he could remarry.
However, Rav Moshe does add that if the woman would show up at some point that he should write her a get immediately and pay her her ketubah.