A letter arrives from Mr. Collins. He offers condolences, but spends most of the letter underlining how Lydia's heinous offense will ruin the other Bennet girls' chances at marriage. Mr. Collins adds that Lady Catherine agrees with him.
Lydia's actions are an extreme breach of rank and polite behavior, both of which are extremely important to Mr. Collins and Lady Catherine.
More time passes, but all attempts to find Wickham and Lydia fail, and Mr. Bennet returns home. He asks Elizabeth not to talk with him about Lydia, saying that he brought this on and only he should suffer.
Mr. Bennet realizes his fault in the matter, but tries to remain isolated from his family. He doesn't realize that everyone will suffer, not just him.