Upon hearing the edict, Mordecai and every other Jew fell into deep mourning, on account that it just became legal to kill them and all.
When Esther heard that Mordecai was going around in sackcloth and ashes, she sent a servant to find out what was wrong. In reply, Mordecai sent her up a copy of the edict and a message about how much Haman had paid for the privilege of murdering all the Jews, begging her to ask the king to reverse it.
Esther sent back a message saying she was sorry, but she couldn’t go to the king unbidden or her life would be forfeit, and well, he hadn’t called for her in over a month.
Mordecai sent word reminding her that she too was Jewish, which was bound to be found out, and so the edict applied to her as well.
So Esther told Mordecai to tell all the Jews to fast for three days and then she would try her luck with the king.