When did Chizkiyah get sick?
Rashi (from Seder Olam 23): It was three days before the fall of Sancheriv. On the third day he went to Beis Hashem to pray, and Sancheriv fell; it was the [first] Yom Tov of Pesach (Shir ha'Shirim Rabah 1:12). Rashi (6) - when Hashem told him (verse 6) that he will live (recover from his illness), he promised also to save him from Melech Ashur.
Why did he say both "Mes Atah" and "v'Lo Sichyeh"?
Rashi citing Brachos 10a: You will die in this world, and not live in the world to come, for you did not marry 1 (engage in Peru u'Rvu) 2 .
Radak, Targum Yonasan: You will not be healed from it; you will die amidst the illness.
Malbim (Yeshayah 38:1): Yeshayah hinted to him that one has three enemies. (a) Other people - they are the most external. (b) Less external are his bodily powers. When they do not function properly, he is prone to die. (c) Most internal are His bad Midos and lusts. He can tilt them with his choice. When he does not, they make his Nefesh ill and bring his demise. Now, the two external enemies were threatening him. When a person fixes the most internal enemy and improves his deeds, also the external enemies are subdued in front of him.
Brachos 10a: Chizkiyah refrained, for he saw through Ru'ach ha'Kodesh that unworthy children would come from him. Yeshayah rebuked him. Do not delve into Hashem's secrets! Do what you are commanded, and Hashem will do what He wants! Chizkiyah accepted this.
Radak: Indeed, his son was born after this. [He lived another 15 years, and] Menasheh was 12 when [Chizkiyah died] and he became king.
