What did the king of Nineveh do when he heard Jonah's message?

Answer: Sat in sackcloth and ashes

The king rose from his throne, took off his royal robe, covered himself with sackcloth and sat in ashes.

“When word reached the king of Nineveh, he got up from his throne, took off his royal robe, covered himself with sackcloth, and sat in ashes.”— Jonah 3:6 (BSB)