What happened to the city of Allendale in "There Will Come Soft Rains" by Ray Bradbury?

In this story, Bradbury does not explain in any detail what has happened to the city of Allendale. Instead, he gives descriptive clues to make the reader aware that there has been a nuclear blast in the city. This house is one of the few survivors of the blast. Outside, the blast is evident all around: one side of the house is "charred," for example, and there are charred silhouettes of some of the victims. Another survivor, the family dog, is covered with sores caused, presumably, by radiation sickness.
By not explicitly describing what has occurred in Allendale, Bradbury is able to focus on his central message: that we must not rely too heavily on technology, because technology is capable of considerable destruction. Bradbury really emphasizes this point in the closing lines of the story, in which the reader is left with the image of a ruined, burned-out house. The only survivor is a talking wall, a symbol of man's over-reliance on technology and his desire for convenience.

Allendale is the fictional name given to the city where the automated house—the main character of Ray Bradbury's short story "There Will Come Soft Rains"—is located. The house reveals the name of the city in its morning announcements, which seem to fall on deaf ears because no human characters are present. Despite the absence of humans, the house carries out its duties, including making breakfast and then launching into a daily cleaning routine. Eventually, the reader discovers the human characters have been killed in an atomic bomb blast which, miraculously, has spared this one dwelling:

The house stood alone in a city of rubble and ashes. This was the one house left standing. At night the ruined city gave off a radioactive glow which could be seen for miles.

Allendale has been the victim of a nuclear attack. Bradbury first published the story in 1950, five years after the end of World War II and the bomb blasts which destroyed the Japanese cities of Hiroshima and Nagasaki, but his story is definitely a vision of the future where human beings have been rendered virtually obsolete in the running of their own house. The story mirrors the post-apocalyptic theme of Sara Teasdale's poem of the same name that was published in the years after World War I.