Hezekiah's father Ahaz didn't trust God.  He turned to the Egyptians because they had chariots and horses.  Sennacherib--the king of the Assyrians (A violent country, just as it--Iraq--is today) was stalled, on the northern border of Jerusalem as they were getting ready to invade.

An interesting thing about the account--given in several books of the Old Testament--is that it is also validated in archeological history.  Sennacherib wiped out 28 cities before he laid siege to Jerusalem. 

In the British Museum in London they have the actual carved walls that lined Sennacherib's palace in Assyria.  I've been to the British Museum, seen the carved panels (which are dozens upon dozens of feet long on both sides of the museum wall) and they are impressive.  Every time Sennacherib defeated a city, he had his artists carve a rendition of the events of the battles in the order they happened onto the palace halls.

Interesting to note that the accounts in the Bible are one-hundred percent validated by archeology. But then, we know the Bible always speaks truth.

By the time Sennacherib was sitting outside ready to invade, Ahaz was dead and Hezekiah was king.  Hezekiah turned to God instead of Egypt.  The Assyrians were wiped out as they waited to invade, "...one hundred and fourscore and five thousand died in the night."  God always wins.