So we're in 2004 and we see an animated version of Izumi Konata, standing on the side of a busy street in Japan, she seems to be looking at her cell phone for directions. Behind her, there is a truck, and Izumi Konata doesn't see it coming, and it hits her. The driver stops and runs over to her body and calls 911 for help. The police and the ambulance arrive, and Izumi Konata survives, but is rushed to the hospital, where they find that the truck had hit her in the head very hard, and that she will have to have head surgery, the only surgeons who can perform this kind of surgery live in the United States. Izumi Konata's parents take her to the United States, and a long and expensive head surgery is performed. However, something very strange happens after the surgery. Izumi Konata wakes up after the surgery, and finds that she can't remember anything... and she finds that she can only speak English... she no longer knows Japanese... and she can no longer talk about video games, or cosplay, or manga, or any of the things that made her who she is... she is a completely different person now...
... the punch line is that, now Izumi Konata is a completely different person, she has lost everything that made her who she was, and she only has a vague memories of who she used to be, but the Izumi Konata that her friends loved, and that they grew up with... is gone forever, and she will never be able to come back.