Quick answer
Repeated correction refused, and then a break that cannot be repaired. The verse is about accumulation, not about one bad decision.
Read both lines
Often rebuked is the load-bearing phrase. This man has been told, more than once, by people who bothered, and the hardening is his answer to them. The idiom is stiffening the neck, the same picture used of Israel in the wilderness, and it describes a physical refusal to turn the head toward something.
The second line's force is in its two qualifiers. Suddenly, because the breaking looks abrupt from outside even though the process was long, and without remedy, וְאֵין מַרְפֵּא, no healing. This book normally leaves a door open. Here it says a point exists past which the door is not there, and it does not say where the point is, which is the warning. The chapter opens on this and closes 26 verses later on two kinds of people who cannot stand each other, so the book's last couplet chapter begins by describing how someone becomes unreachable.
Related: Proverbs 28:14 · Proverbs 6:15 · Proverbs 13:1 · Proverbs 15:10
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.