Hezekiah, son of King Ahaz, and mother Abijah, became king at age 25. He reigned 29 years. His reign was said to be a good one. He took the throne during a crucial time in Judah's history. Assyria had recently conquered Aram. And during the early part of his reign, the Assyrians conquered the northern kingdom of Israel. And, Judah itself had to pay an annual tribute to Assyria.
The name Hezekiah is the English translation of the Hebrew חִזְקִיָּה, which means "strengthened of Yah."
With the encouragement of the prophet Isaiah, Hezekiah undertook a religious reform, in removing idolatrous elements from worship, cleansing and sanctifying the Temple, and restoring the Levites to their Temple duties. He sent letters across Judah and Israel to have the people attend a Passover celebration. The celebration was a huge success. In fact Jerusalem hadn't seen a celebration like that since the days of King Solomon.
Hezekiah strengthened Judah politically, expanded its borders, and had the people tithe their crops. He built an underground tunnel to bring water into Jerusalem, in case of a siege. Later, King Sennacherib invaded Judah, surrounded Jerusalem, and sent Hezekiah a letter, stating, basically, that Judah should surrender, claiming that the Assyrian gods were stronger than the Lord.
Hezekiah and Isaiah prayed to the Lord. God answered Hezekiah and Isaiah by saying that He would defend and save the city. That night, 185,000 Assyrian troops mysteriously died and their bodies were seen all across the landscape in the morning.
Later, the Lord healed Hezekiah of a deathly sickness, after Hezekiah had prayed, and added 15 years to his life. When he died, his son Manasseh became the new king.
The tunnel Hezekiah built is still in existence today. A stone inscription was found describing how King Hezekiah's tunnel was dug by 2 teams of miners, starting at opposite ends, and meeting in the middle. There is also a clay Hebrew seal impression found, with Hezekiah's name on it, now in the Israel museum.