Albert Barnes Commentary Isaiah 36:21

"But they held their peace, and answered him not a word; for the king`s commandment was, saying, Answer him not." — Isaiah 36:21 (ASV)

But they remained silent – Hezekiah had commanded them not to answer. They were simply to hear what Rabshakeh had to propose, and to report to him, that he might decide on what course to pursue. It was also a case in which it was in every way proper that they should be silent. There was so much insolence, self-confidence, and blasphemy, the proposals were so degrading, and the claims were so arrogant, that it was not proper for them to enter into discussion or listen for a moment to the terms proposed.

Their minds were also so horror-stricken by the language of insolence and blasphemy, and their hearts so pained by the city's circumstances, that they would not feel inclined to reply to him. There are circumstances when it is proper to maintain a profound silence in the presence of revilers and blasphemers, and when we should withdraw from them and go and spread the case before the Lord. This was done here (Isaiah 37:1), and the result showed that this was the course of wisdom.