Isaiah 23:15
And it shall come to pass in that day, that Tyre shall be forgotten seventy years, according to the days of one king: after the end of seventy years shall Tyre sing as an harlot. And it shall come to pass in that day, that Tyre shall be forgotten seventy years, according to the days of one king: after the end of seventy years shall Tyre sing as an harlot.
The verse belongs to the first part of Isaiah, chapters 1 to 39, made up of distinct single prophecies given at different times and on different occasions during Isaiah's active public work as God's messenger among rulers and people and afterward collected together, rather than to the second part, chapters 40 to 66, given in his later years apart from active work and occupied wholly with the future, including deliverance from Babylonian captivity under the monarch Jehovah would send and deliverance from sin and error through the Messiah and His spiritual kingdom.
It belongs to the prophecy of Isaiah 23 foretelling the destruction of Tyre by Nebuchadnezzar, which is separate from the later destruction of Tyre by Alexander when its stones and timber and dust were thrown into the sea to make a bridge to New Tyre.
To be forgotten seventy years means to be counted for that period as a city of no importance, and Tyre's humiliation came through Babylon.
The verse presents Tyre as left desolate for the seventy years.
The phrase according to the days of one king gives the seventy years as matching the lifetime of the king who destroyed Tyre, namely Nebuchadnezzar's lifetime.
The same phrase the days of one king is also understood as the duration of one kingdom, namely the duration of the Babylonian kingdom.
At the close of the seventy years the Medes and Persians overthrew Babylon, giving Tyre a chance to revive and an opportunity to rebuild, and after the seventy years Tyre would be restored as a prosperous city, which was fulfilled.
After its restoration Tyre would return to its former wickedness, pictured as singing as an harlot and committing fornication with the kings of the earth, and having used its wealth for evil rather than for God's glory, its restored character makes it a type of the apostate church.
Its visitation after the desolation means God would do a work for the city, fulfilled when a Christian church was later raised up in Tyre, though the city did not escape the punishment due it.