Lech Lecha

Our parsha presents the inability of our matriarch Sarah to have children. Why were the matriarchs barren? For God desires their prayers and looks forward to their conversations… He said: They are beautiful, they are rich; if they have children, when will I hear from them? (Midrash – Breishit Rabba 45:4) 
The midrash tells us further that not only was Sarah barren she didn't even have a womb making it impossible for her to conceive.
Rav Kahaneman asks why was it that Hashem had to make her without a womb? If He wanted her teffilos, wouldn't making her barren be enough? 

The parsha also relates the saga of Avraham and Sarah going down to Egypt where Avraham tells Sarah to say  אמרי נא אחותי את למען ייטב לי בעבורך say that you are my sister so that it will be good for me. Rashi says so that they will give me presents. What kind of person would let his wife be taken away so that things will be good for him? In essence, he is saying, “let the Egyptians take you and have their way with you as long as I'll be Ok.” Also, since when does Avraham take presents from kings ? He refused to take anything from the king of Sedom. 

On this last question, the בכורי אברהם answers that we are told מעשי אבות סימן לבנים , that what happened to our forefathers is a sign for the children; what happened to them happens to us. Just like Avraham went down to Mitzraim, so, too, did his children; just like Pharaoh was afflicted now, so would the Egyptians be later. What Avraham wanted was that his children should get gifts when they left Egypt, so, therefore, he wanted gifts. 

The Zohar writes that Avraham was not worried about Sarah getting harmed by the Egyptians because he saw that Sarah had an angel in front of her at all times, so he knew that, through miraculous ways, nothing would happen to her. He was only worried that he shouldn't get harmed because he was not on that level, so he had to ensure that he would be saved naturally. 

With these two answers, we can understand Rav Kahaneman's answer to why Sarah had to not only be barren but also have no womb. He answered that מעשי אבות סימן לבנים , for there were many periods of time throughout the history of our nation that all seemed lost ,when either spiritually of physically. Our nation seemed doomed and that by all natural means the Jewish people would cease to exist. Therefore, Hashem made it that Sarah had no womb for us to recognize that we would be able to survive even when it's impossible . 
Perhaps that's why, too, Sarah had to go through being abducted by the king,  surviving and coming out unscathed by way of miracle to ensure that the Jewish people, throughout the generations, will be able to survive the various exiles even if only by way of a miracle. 

Rav Kahanamen said that when his grandfather, the Ponevezher Rav, came to Israel after the war and wanted to rebuild, everyone told him it was impossible. He would never get back what we had; it'll never happen. Look what we have all just been through; it's over. He answered, that’s correct. We can't rebuild naturally but the Jewish people are born out of miracles and will survive with miracles. Look at the world now; there are probably more yeshivos than ever in history. 
We have to realize that nothing is impossible. No matter what’s going on and what one might think, we are a nation of survivors. Whether through natural means or by miracle, we have the ability to overcome any adversity and survive! ‎