When Yosef revealed himself, "Ani Yosef -- ha'od avi chai?" the shevatim were too stunned to answer. Chazal see this as a model of how we will react when G-d reproves us on the day of judgment. If the great shevatim could not answer the reproof of their brother, surely we will be struck dumb when G-d reveals the truth of what we should have done with life to us.
Chazal refer to Yosef's words as reproof, rebuke, but the meforshim struggle to see how that description fits. Yosef didn't give his brother's a musar shmooz and tear into them. To the contrary, he just revealed who his is as a statement of fact. R' Chaim Shmuelevitz in the Sichos Musar suggests that this is what real reproof consists us -- a revelation of truth that undermines all false assumptions. Imagine a scientist who constructs an elaborate theory as to how and when species X went extinct. You can argue from today till tomorrow whether the theory is right, but if someone walks into with a living specimen of species X, all bets are off. That's tochacha. The brothers theorized that Yosef would amount to no good. "Ani Yosef" -- I am Yosef the tzadik, even though I have been in Egypt. You declared my Torah extinct and my religiosity extinct, but here I am.
The Netziv points out that Yosef asks, "Ha'od *avi* chai?" -- Is *my* father still alive. Yosef was telling the brothers that although Ya'akov was the father of the entire family, he had a unique relationship with each one of his children. To think that getting rid of even one brother would make no difference -- after all, there were 11 others -- missed that point. How could *my* father continue to live absent that special relationship all these years?
Rav Charlap (the son of Rav Y"M Charlap) says an amazing pshat in this pasuk. He suggests that it is not speaking about Ya'akov at all. We know that when the brothers came to Egypt, they did not recognize Yosef, but he was able to recognized them. This was by Divine design in order so that Yosef would be able to bring about the fulfillment of his dreams. Yosef, however, did not know that. He wondered the entire time how it was possible that his brothers could be around him and fail to recognize him. After all, Rashi tells us (37:2) that he resembled their father Ya'akov Avinu! Yosef wondered to himself whether he had lost his yiddishe panim, the dmus d'yukno, the spiritual resemblance to his father. Perhaps he had been influenced too greatly by Egyptian society. When he now reveals himself, he asks his brothers, "Ha'od avi chai," is the semblance to my father that I left home with still alive within me? Do I still look like one of the shivtei K-h, like Ya'akov Avinu?
At this point there was no longer a need for Divine intervention to keep up the charade, and so the mask was off. The brothers now saw in the face of Yosef that this was indeed their brother, that he still retained the resemblance, spiritually as well as physically, so their father. They could not answer, "ki nivhalu mi'panav," because they were astounded at his face, the face they now recognized.