I have included a link below to the enotes page outlining the biography of Alice Sebold, which should help you investigate her life further. The short answer to this question is that yes, it was. It is always difficult to trace how much fiction is impacted by fact or real events, but a fundamental event that occurred in the author's own life was when she was raped and beaten. Later it emerged that the man that raped her had raped and killed a girl before her in Norristown in Philadelphia. Other elements that are derived from this true event are the corn field, which is still used as a short cut by kids who attend Norristown High.
Apart from this basis in fact, the rest of the story is made up. The story does however raise interesting questions about the relationship between fact and fiction. Is it possible to write a story convincingly about such a traumatic event if you have not experienced it yourself? To what extent is writing a kind of therapy for the author?
Below is a link to a biographical entry written about Alice Sebold. In short, yes, The Lovely Bones was based on true life events: Sebold was brutally raped in a tunnel while she was in college. Once she reported the crime, the police told her of another rape that had happened in the same tunnel in which the victim had been dismembered. She was lucky to be alive, but, understandably, had difficulty moving past this traumatic event. The Lovely Bones was most likely an opportunity for Sebold to at least attempt to make peace with herself regarding this incident. Writing the novel, reliving these events probably provided some release for these pent up emotions.
Also written in her biography is the tension that she often felt with her parents, her mother in particular. It's easy to see that there were similar issues with the surviving daughter and the mother in The Lovely Bones as well. Specifically, Sebold's mother suffered from anxiety and alcoholism, so the children did not live with her; Abigail Salmon (mother in the novel) eventually abandons her family because she cannot deal with the death of her daughter.
While the story is clearly fictionalized, Sebold used actual events, and probably some of her own emotions, for the basis of this novel.
The Lovely Bones movie was ambitiously directed by Peter Jackson, who brought us The Lord Of The Rings and Hobbit trilogies. In the movie, the pedophile character, George Harvey, rapes, murders, and dismembers 14 year old Susie Salmon; Susie is lured into an underground den when she takes a shortcut home from school one day. The story is set in the year 1973. While George Harvey is a fictional pedophile, his profile is a composite of many of the nation's most heinous child predators. His victims, including Susie Salmon, are mostly young girls and female teenagers (again, all the victims are fictional composites of actual serial killer victims).
The author, Alice Sebold, was herself a victim of rape. While returning to her college dormitory room one evening, she was brutally assaulted and raped in a tunnel to an amphitheater. She was later to find out that a young woman had been murdered and dismembered in that same tunnel. Later, Sebold took the stand at her rape trial, and the rapist was given the maximum sentence for rape and sodomy. So, you could say that, for Alice Sebold, The Lovely Bones was personally cathartic. The novel started out as Monsters, and was briefly named This Wide Heaven before it became The Lovely Bones. Writing provided an avenue for Sebold to deal with her own pain and anguish. At the same time that she acknowledges that her novel is a work of fiction, Sebold allows for the fact that her readers will be tempted to explore the link between the novel and her own rape. She realizes that it is only natural that her readers would want to make sense of the connection.