According to the author of 2 Kings, God tells Hezekiah that He has heard his prayers and seen his tears, and promises to make him better. In three days, Hezekiah will feel well enough to visit God's house.
2 Kings 20:5: Turn again, and tell Hezekiah the captain of my people, Thus saith the LORD, the God of David thy father, I have heard thy prayer, I have seen thy tears: behold, I will heal thee: on the third day thou shalt go up unto the house of the LORD.
This verse states that God does not change His mind, which may seem to contradict God's decision to extend Hezekiah's life in 2 Kings 20:5 after initially telling him he would die.
Numbers 23:19: God [is] not a man, that he should lie; neither the son of man, that he should repent: hath he said, and shall he not do [it]? or hath he spoken, and shall he not make it good?