In those days was Hezekiah sick unto death. And Isaiah the prophet the son of Amoz came unto him, and said unto him, Thus saith the LORD, Set thine house in order: for thou shalt die, and not live.
But see notes on Isaiah 36:1-22; Isaiah 37:1-38, in which the theory is advanced by Hincks, that Hezekiah's sickness is out of its right order in our collocation of Isaiah's text, and that it really came about the 14th year of his reign, when Sargon (not "Sennacherib," Isaiah 36:1) the "king of Assyria" came up against Palestine (Isaiah 20:1, etc.; 36:1). So "the 14th year" in Isaiah 36:1 is out of its place, and refers to Sargon's futile invasion, at least eleven years before Sennacherib's invasion.
Set thine house in order - Make arrangement as to the succession to the throne; because he had no son then: and as to thy other concerns.
Thou shalt die - speaking according to the ordinary course of the disease. His being spared fifteen years was not a change in God's mind, but an illustration of God's dealings being unchangeably regulated by the state of man in relation to Him.