Quick answer
Deafness comes back around. The verse is a warning aimed at the comfortable, and its subject is the man who stopped listening rather than the God who answers.
Read both lines
The first verb is deliberate and physical, stopping the ear, so this is not a man who failed to notice but one who shut something. The word for the poor man's cry is the cry of distress, the same register as a call for help, and the verse assumes it was audible. Then the second line hands the man the identical position, crying and unanswered, and leaves the agent unnamed. That silence is the craft. It does not say God refuses him and it does not say his neighbors do; it says the posture he chose is the posture he receives, which is how this book normally describes the world.
What it does not mean
This is not a mechanism for how prayer works, and it must not be read as God declining to hear someone. Prayer in the New Covenant is communion rather than a transaction, and no proverb converts it into an account with conditions attached. The verse belongs to the chapter's justice thread, alongside 21:3 and 21:15, and it is aimed at a person with the power to help and the option of not hearing. Aimed anywhere else it does damage: it is not an explanation for why anyone's prayers feel unanswered, and it is not a tool for auditing whether a suffering person gave enough.
Related: Proverbs 21:3 · Proverbs 19:17 · Proverbs 13:23 · Proverbs 14:31
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.