Now, after a remarkable recovery, she can talk again and has begun to regain her memory, sending her father "from despair to joy."
Sarah Scantlin's family and friends celebrated the development on Saturday at the health care center where she lives.
"She's 100 percent Sarah again. The family is back together, and it's just simply a joyous situation," her father, Jim Scantlin, said in a broadcast interview.
Scantlin's father says he knows she will never fully recover, but her newfound ability to speak and her returning memories have given him his daughter back.
For years, she could only blink her eyes — one blink for "no," two blinks for "yes" — to respond to questions that no one knew for sure she understood.
"I am astonished how primal communication is. It is a key element of humanity," Scantlin said, blinking back tears.
She still suffers constantly from the effects of the accident. She habitually crosses her arms across her chest, her fists clenched under her chin.
Her legs constantly spasm and thrash. Her right foot is so twisted it is almost reversed. Her neck muscles are so constricted she cannot swallow to eat.
Scantlin was 18 when she was struck while walking to her car in 1984. She had been aware of her surroundings but unable to make any sounds other than loud crying until a month ago, when she told staff members, "OK, OK."
"It just happened one day and nobody really knows why," said Sharon Kuepker, administrator for the Golden Plains Health Care Center.
She is now forming other words, counting and remembering people and places, staff members said.
"You condition yourself to be able to try to deal with something like this, and then all of the sudden, the world instantly changed from despair to joy because it's amazing how important communication is between human beings," her father said.
The driver who struck Scantlin, Douglas Doman II, served six months in jail after being convicted of driving under the influence and leaving the scene of an injury accident.
Sarah's father told The Early Show co-anchor Harry Smith in a separate interview that his daughter had been "basically unresponsive, in the regular sense of the word 'alert.' She was able to do eye contact and was very aware of her surroundings, but otherwise, physically, she was completely incapable of doing anything."
Sarah's mother, Betsy Scantlin, says Sarah actually started to make some noises a couple of years ago. "We didn't have any idea what that meant," she said to Smith. "We just knew it was a noise that she hadn't made for 18 years, and the people -- it's kind of like having a baby. You kind of learn to know the sounds, and they could kind of tell whether she wanted her TV changed or she was hurting or something, but otherwise, nothing."
Jim recounted the phone call he and Betsy got, informing them of the unimaginable: "It was amazing. I'm in the living room. Betsy was in the computer area, and the phone rings, and I'm immediately aware that it's the nursing home…where (Sarah) resides.
"And suddenly, I'm aware that there's a profound, distinct difference. Rather than speaking about Sarah, it became very clear she was speaking to Sarah. It was the most amazing feeling in the world."
Besty says she's "still stunned" to be hearing Sarah's voice again. "There's just no words. Twenty years ago, I cried a lot. This week, all I've done is laugh because, when I heard her say, 'Hi Mom,' I said, 'Sarah, is that you?' And she said, 'Yeah.' And all I can do is just say -- I've just laughed ever since, because it's just so amazing."
What's more, Jim says Sarah is showing "uncanny recall. It's amazing how -- considering how severe and profound her injury was, her ability to recall -- it's astonishing."
Sarah's doctor, Bradley Scheel, adds that, "It's just been amazing. We're all thrilled. And it seems, almost from day to day, she's able to make more responses. We're really anxious to see how she does from here."
Scheel said physicians are not sure why she suddenly began talking but believe critical pathways in the brain may have regenerated.
"It is extremely unusual to see something like this happen," Scheel said.