Wonder Woman was killed by Neron, yes. Back in Messner-Leob's run, Hippolyta had called for another contest for the title of Wonder Woman, and rigged it so that Artemis would win, because Hippolyta had received a prophecy saying that someone in the Wonder Woman suit would die soon. Anyone remember that? No?

Well anyway, Byrne's run came along, and it was revealed that Hippolyta accomplished this with a spell (commissioned from another Amazon, later revealed to be corrupt) that transferred Diana's power to Artemis when they were near. More plot happens. Everyone (Diana, Artemis, Hippolyta, Cassie, random supporting cast cop) ends up in hell. Neron takes advantage of Diana's current weakness to kill her.
Then her gods turn her into a goddess, and then eventually turn her into a mortal when she's not a very good goddess. (cares too much about her friends) Technically I guess she was dead for an issue or so before turning into a goddess, but if you count all the time that she was a goddess as not being alive in the mortal sense, then she was "dead" for a very long time. Longer than Superman.
So now you know.