What does Isaiah 37:15 mean?

Hezekiah has received a letter from Sennacherib (Isaiah 37:10) that skillfully promises to destroy the king, Jerusalem, and Judah if they trust the Lord. The Assyrians want Hezekiah to surrender before an attack begins. At this point, It's possible Hezekiah had begun to think his alliance with the Egyptians had paid off (2 Kings 18:19–21). The news was that Sennacherib's attack on Jerusalem had been delayed by the arrival of a second wave of Egyptians to attack him (Isaiah 37:8–9). The letter from Sennacherib made it clear that this was far from over. The threat of utter destruction remained real and imminent. The possibility of surrender remained on the table. The weight of Hezekiah's burden as king must have been enormous.

Unlike so many of the kings of Judah and Israel before him, Hezekiah takes the only immediate action that makes any sense: He prays. He turns immediately to the Lord God of Israel and lays the devastating letter out before his God (Isaiah 37:14).

This is what Hezekiah's father, Ahaz, refused to do. Like Hezekiah, he had a plan to solve an overwhelming problem. In that time, Syria and the northern ten tribes of Israel had joined forces and were coming to destroy Judah. To resolve the situation, Ahaz had made a terrible and foolish alliance with the Assyrians, the greatest threat in the world. The Lord sent Isaiah to tell Ahaz that the Lord, not human effort, would solve the threat from the Israel/Syria alliance. God even offered to show Ahaz a miraculous sign to prove that He would protect Judah. Ahaz refused. He would trust his own plans and not the Lord (Isaiah 7).

Perhaps Hezekiah learned from his father's failure. Instead of refusing the Lord, he asked for God's help when he needed it most.