Jehoash, King of Judah (not the same man as Jehoash, King of Israel, who lived at the same time) was the son of King Ahaziah. His uncles had been assassinated when he was a baby when Jehu, king of Israel's troops killed all 42 of Ahaziah's brothers. When Ahaziah himself was soon afterwards assassinated by Jehu, Queen Mother Athaliah exterminated the entire succession, all her grandsons, and assumed the throne of Judah herself.
Jehoash was the only survivor of the purge, saved by his aunt, Jehosheba, the wife of High Priest Jehoiada, and hidden in the palace sleeping quarters. He was smuggled to the Temple and raised in secret for the next six years. At then end of this period Jehoiada proclaimed him king and had Queen Athaliah executed. Jehoiada was regent while Jehoash was a minor, but when Jehoash grew up he reigned in his own right and restored the Temple. But he later became impious and corrupt and was assassinated by Jozacar and Jehozabad, and was succeeded by his son Amaziah.
The history is found in the Bible, II Kings 11 and II Chronicles 24.