“I said, “In the prime of my life I shall go to the gates of Sheol; I am deprived of the remainder of my years.”  I said, “I shall not see Yah,

The Lord in the land of the living; I shall observe man no more among the inhabitants of the world. Isaiah 38:10-11

Hezekiah was sick and near death, and the uncertainty of seeing the Lord weighed on him deeply. Hezekiah was an Old Testament believer who did not know Jesus and what He had done for us on the cross. A true believer does not need to fear death or be uncertain about seeing the Lord.  

However, a non-believer is already dead in tresspasses, and their fate is hell.