Such a case is certainly a tragedy, but in the language of Shulchan Aruch is NOT an “aguna”. “
It most certainly is today. It means any woman who is not free to re-marry. Most commonly it was that men went off to war or merchant vessels, and were missing in action or lost at sea, but with no body recovered, thus rendering it assur for them to re-marry. Their wives were known as agunos. But it has come to also mean ANY woman who cannot halachically remarry.