In general, a highly traumatic experience is: dramatic;
you feel isolated; and
you don't understand what's happening.
In a situation like this, the body's neurological system automatically allows you to do several things so that you can try to survive. You might fight back. Or try to escape, or flee. This makes up the "fight or flight" reaction you always hear about.
If you can't do either of these (are you too small? is there even a way out?), your body tells you to freeze—either physically or psychologically. If you have seen an animal freeze in front of a car, this is what is happening to it. By freezing, the body hopes that the bad thing won't notice you and will leave you alone.
If the bad thing happens but you can defend yourself or run away, you might have a memory of the event. But, you will generally be ok.
If you freeze and the bad thing happens, the energy of that initial trauma can get locked up in your body. Your brain also stays on high alert so you can be prepared if something like this happens again.
This is a good evolutionary system, but it's not perfect: the parts of the brain that are involved with this process don't know the difference between the bad thing happening, or you thinking about the bad thing. Both keep your body in a high state of stress.
This is called Post Traumatic Stress Disorder. Being in a high state of stress over time can be bad for your physical health and happiness.
Emotional Freedom Techniques
With professional help, you can gently and easily release past trauma with EFT. It works for little issues and for things as big as Post Traumatic Stress Disorder.