פירוש על מלכים ב 20:1

In those days. [This occurred] three days before Sancheiriv's downfall.¹Alternatively, the entire episode is stated in chronological order, and it occurred after Sancheiriv's downfall. The assurance Chizkiyohu requested that he will be saved from Ashur mentioned here refers to future attacks. Although Ashur had been soundly defeated, Sancheiriv's successors might soon try again to attack.—Ralbag

In those days Hezekiah was sick unto death: I should think that this was after the preceding story. And don't be surprised when it says, "I will deliver you out of the hand of the King of Assyria," (2 Kings 20:6) because it is possible that Esarhaddon threatening Jerusalem.

Chizkiyohu became … ill. On the third day, when he went up to the Beis [Hamikdosh] of A, was the day of Sancheiriv's downfall, and it was on the Pesach holiday.²To commemorate this event, reference to Sancheiriv's downfall is mentioned in the Hagaddah on both Seder nights. On the first Seder night it is mentioned in ובכן ויהי בחצי הלילה, and on the second Seder night it is mentioned in ובכן ואמרתם זבח פסח.

For you are going to die. On this world.

And you will not live. In the World to Come because you have not married as is [explained] in [Maseches] Berachos.³10a. Chizkiyohu refused to marry and have children because it was revealed to him that he would have a very wicked son, and a period of extreme corruption would follow.