I suppose you are refering to the inciting incident that caused Willy to commit suicide? Clearly there are a number of different events that work together to prompt Willy to take his own life, forcing him to realise with disturbing frankness the massive failure that his life has been compared to the hopes and dreams that he had initially had. However, I would want to argue that the biggest event that is of crucial importance in Willy's decision to take his own life is getting fired from his precious job at the beginning of Act II. This is clearly crucial, because so much of Willy's identity and reputation is bound up in his job and his supposed success as a salesman. Getting fired means that he has to confront his failure in his profession, and can't hide behind his elaborate fabrications any longer. The irony of it is that his boss, Howard, thinks in some strange way that this will be good for Willy:
Whenever you can this week, stopy by and drop off the samples. You'll feel better, Willy, and then come back and we'll talk. Pull yourself together, kid, there's people outside.
He does not understand how intrinsic Willy's position and work is to his identity, and what a devastating impact being fired will have on his life. It is this event that turns Willy desperately to thinking of ways in which he could make his life a success, which makes him think of killing himself and gaining money in exchange for his life.