Even though Hezekiah has been completely faithful to the Lord, an army from Assyria shows up led by their king, Sennacherib. Sennacherib runs a tight propaganda campaign, yelling in the language of Judah about how the battle's all but won already and how their god isn't going to be able to protect them just like all the other gods of other nations were unable to stop the mighty Assyrian juggernaut and so on and so forth.
Hezekaih heard about their coming beforehand though so he had time to take a few precautions. He had the walls of Jerusalem repaired and improved and stopped up all the water sources outside the city. When Sennacherib tried to demoralize the army, Hezekiah responded that all those other gods were made by the hands of men and that Assyria has certainly never faced a God like the theirs. He told his own army that, with God really on their side, they wouldn't even have to fight at all.
Reading through this part, I was interested to see how God would feel towards Hezekiah afterwords. I sense a kind of tension from the previous stories about relying on God. It's obviously insulting to God to sit back idly and demand God do everything to make life easy for them, but on the other hand, attempts by ancient kings to fix things themselves seemed to also offend God. David's census of the men in Israel is an example of that. I still can't fathom why it was such a terrible thing for him to have done. There's another king who fell victim to this also, although his name and the specifics of his story escape me. I believe he set up a trade agreement with Tyre or someone and they built a bunch of ships together, but they didn't consult with God first so God destroyed their plans. Hezakiah is very devout though, so he probably spoke at length with his prophets off-camera before working on the walls and the water sources to prepare for the siege.
In the end, Hezekiah yells prayers and God sends an angel to wipe out the Assyrian army. With the battle over, the Sennacherib can do nothing but go back to Assyria in shame.
God performed many miracles for Hezekiah. He eventually got a little prideful because of it and stopped being very thankful. God reprimanded him though and Hezekiah quickly snapped back in line. Later on, God left Hezekiah to test him (I'm not sure what exactly this means) but Hezekiah remained faithful. It sounds like there's a lot of this story that we're not hearing because it's covered in the Book of the Kings of Judah, but it sounds like Hezekiah is just about the most virtuous and faithful king we've heard about to date, and Judah thrives under him. Babylon even starts sending down messengers to learn about what was making Judah so awesome.
Hezekiah's son Manasseh however, took the throne at 12 years old and adopted the customs and gods of all the other nations around Judah. There's an interesting line in 39:9 that says "They did more evil things than the nations the LORD had destroyed to make room for the people of Israel." So Assyrians show up again and this time take Manasseh captive. While a prisoner, Manasseh humbles himself and prays to God, and God takes pity on him, returning him home. After that he straightens up and worships the Lord. Manasseh ruled for 55 years.
Amnon ruled next for 22 years. He was king type 1, the bad kind. He was eventually killed in a plot by his officials. Then the people of Judah killed the officials and made Amnon's son Josiah king. So, that was kind of a mess.
Josiah became king at 8 and he must have had some really good advisers because he became a type 2 king, the King David type. He tore down the altars to other gods, cleaned out the temple, etc, etc. When they were clearing out all the treasure from the temple vault to get some restoration projects going, someone stumbled across the scroll of the law given to Moses, which I guess had been lost for a while. When Josiah saw it he flipped out. Even with all the good Josiah had been doing, nobody had really been following these rules. He consulted with some prophets and God told Josiah that he was doing a good job and had made himself humble before God, so he was going to be okay, but yes, God was super-angry about the neglect of the law over the last several (hundred?) years, so He was going to basically wipe this country off the face of the Earth. He would wait until after Josiah died though.