What did Hezekiah do with the threatening letter from Sennacherib's messengers?

Answer: Spread it out before the LORD

Hezekiah read the letter, went up to the house of the LORD, spread it out before the LORD, and prayed.

“So Hezekiah received the letter from the messengers, read it, and went up to the house of the LORD and spread it out before the LORD. And Hezekiah prayed to the LORD:”— Isaiah 37:14-15 (BSB)