Main causes of fear
Lack of Socialization
This could happen when the kitten is taken from the mother too soon. The mother cats trains socialization with humans, children, and other cats over a period of time. Kittens should stay with their mom at least for eight weeks; twelve weeks is better.
Your best way to go with a poorly socialized kitten is to get him used to one person in your home at a time, handle him gently, talking to him in a soothing voice, as long as he his comfortable. Allow him to explore his surroundings at his own pace, do not rush the process. Always move slowly around a shy kitty. Once she is comfortable, another person can enter the socialization role.
Cats are creatures of habit and do not like change. They are addicted to their sourroundings and their daily routines. Environmental changes include moving furniture; noise, parties or construction; and introduction of new pets into the household. With any of these changes, it's best to give the cat a safe room for as long as needed. She may stay under the bed for a few hours (or days), coming out to eat and use the litter box. She will gradually become more comfortable and stay out longer periods. The pace varies a lot depending on each cat.
Fear in Cats
When working with fearful cats you should follow this advice: Do not stare directly into your cats eyes when trying to calm her. A direct stare is a sign of aggression among cats.
Fear of People
Unless your cat was born in your home, you don't know what her relationship was like with humans in her life, other than the information provided by the breeder, pet store or shelter. The cat may have fear of humans because of previous abuse.
Your cat can learn to trust humans, starting with yourself, but it will be a slow process. Let her set the pace. Eventually, once she is comfortable and trusts you, other humans in your life may follow the same process. She could remain a one-person cat for the rest of her life.
This would be part of enviornmental changes. More specifically, loud noises, such as construction, parties, and holiday celebrations involving fireworks, such as New Year's Eve and the 4th of July, can be particularly frightening to cats. Since these activities are known of in advance, it is suggested to remove the cats to a safe room with a hiding space until the activities have ended.