One of the immediate aftermaths is likely to be physical harm. She might have suffered from malnutrition, injuries from abuse, or lack of proper medical attention while locked in the basement.
I don't think it is. Usually, stories like 'Locked in the Basement' are made up to thrill and engage the audience rather than being based on real events. There might be elements inspired by real-life situations, but as a whole, it's probably fictional.
One possible reason could be that she was kidnapped by a criminal who wanted to keep her hidden for ransom or other malicious intentions. Another reason might be that she was being held captive by someone with a mental disorder who had an abnormal obsession with her.
If we consider a common scenario in such true stories, it might be that the girl was kidnapped by a deranged individual. The kidnapper would keep her locked in the basement to exert power over her, often for reasons like sexual gratification or just a perverse sense of control. The girl would be in a terrifying situation, with little access to the outside world, proper food, or medical care. She would be constantly living in fear and trauma.
Yes, there are several real - life cases of girls being locked in basements that have received public attention. One of the most infamous was the Elizabeth Fritzl case in Austria. Her father locked her in a basement for 24 years, during which he repeatedly assaulted her and she gave birth to several children. It was a tragic and shocking case that exposed the extreme abuse and control a person can inflict on another.