Following Akeidas Yitzchak, Avraham returned from Har HaMoriah elated, ready to tell to Sarah how Yitzchak had been willing to be tied to the altar and be slaughtered in order to serve Hashem, and was then saved by Hashem's word. Unfortunately, however, he could never tell this to his wife. According to Chazal, when he arrived in Be'er Sheva, he heard that Sarah had gone to Chevron in search of him. Avraham traveled to Chevron and found that Sarah had died and his rejoicing turned to mourning.
Rashi (לבראשית כ"ג:ב' בד"ה לספוד) explains what had happened to Sarah in Avraham's absence; his explanation is a shortened form of a famous Midrash. The Satan, frustrated because he had failed to achieve victory over Avraham or Yitzchak, appeared to Sarah and asked where Yitzchak is. Sarah said that he had gone with his father to study the laws of sacrifices. The Satan answered that this is not true and announced that Yitzchak himself would be the sacrifice. Even though Sarah tried not to believe the Satan, she was troubled by his words and therefore left the house to go find Achiman, Sheishai, and Talmai, three giants who lived in Chevron. She asked them to look into the distance and see if they can find an old man accompanied by his son and two servants. The giants rose to their full height and said they could see the old man on a mountain top holding a knife in his hand while his son is on the altar. Hearing this, Sarah died from the shock.
What exactly was the greatness of Sarah? Chazal say that in a certain sense, the entire country had prospered in her merit. She had spent each of her 127 years living in righteousness, accepting all of Hashem's decrees with happiness. When told, as mentioned above, that Avraham had taken her only son to study, she accepted it without question. Apparently, Sarah's greatness was that she was willing to give up everything for the sake of her son's education. She was also one of the seven known prophetesses, as the Gemara in Megillah (דף י"ד.) says. But Chazal say that she was the greatest of those seven because Hashem spoke to her directly while to the other prophetesses He spoke only through a messenger. She was on a level of prophecy even greater than that of Avraham, as Rashi points out (בראשית כ"א:י"ב). For these reasons, Chazal say that when Shlomo HaMelech composed the song Aishes Chayil (משלי ל"א:י'-ל"א), describing the praises of the virtuous woman, he was alluding to Sarah. All the Pesukim in that song which follow the alphabetical order from Alef to Tav, apply to her, since she fulfilled the will of Hashem from Alef to Tav.