What event occurs that night which provides Montag with an impression of the state of society? What is that impression?
In Part One of Fahrenheit 451, Montag returns home from work to find his wife, Mildred, has taken too many of her sleeping tablets and has suffered an overdose. He calls the emergency services who send two men with machines—not qualified doctors, as Montag assumed they would. When questioned on this, the two men tell Montag that there are so many overdose cases that it is impossible for a doctor to attend each one.
It is this experience which gives Montag the impression of a broken society. Specifically, he feels that the high population of his society has created widespread feelings of isolation and division. This explains why so many people have resorted to overdosing:
There are too many of us, he thought. There are billions of us and that's too many. Nobody knows anyone. Strangers come and violate you.
Mildred's overdose is one of a number of experiences which brings about a complete change in Montag's character. He wants to understand why his society is broken and he starts to believe that the answer might lie in books.
Montag takes enormous pleasure in the first book burning we read about. Everything goes well and he is on a sort of high as he returns home for the evening. Along the way he meets Clarisse which gets him to thinking, something that apparently few people do in the society he lives in.
So when he returns home to find that his wife has attempted suicide by downing an entire bottle of pills, he is somewhat disturbed. Even more disturbing to him are the men that come to help fix the problem who don't even appear concerned. They do it so often that coming to pump the stomach and clear the blood of someone is just another task, like fixing a toilet. This makes Montag really question the state of the society he lives in.