My brakes stopped working and now they're fine…What could be wrong?

So I recently purchased a used car (2004 Honda Civic EX) and it test drove fine. The dealership kept it for a few days to clean it out and detail it. When I picked it up it drove fine and the brakes were working. Then about twenty minutes I was on the highway in bumper to bumper traffic and noticed my car wasn't slowing down when I was trying to brake. I was pressing on it about half way but since it wasn't slowing down I pushed it all the way back and nothing. I ended up rear ending another vehicle. After we exchanged information I drove back to the dealership (on the streets because I was too scared to get back on the highway). The brakes worked fine on the way back and the dealer even tried them himself and they worked fine. They kept the car and said they'd have a specialist come look at it and when I called today he even said they put one hundred miles and the brakes weren't giving them any problem. He probably thinks I'm lying and just wasn't paying attention but I wouldn't be going through all this trouble and going without a car if it wasn't true. What could have caused my brakes to suddenly fail then work fine again?

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