You may find that 'debriefing' or counselling will help you to come to terms with the incident. Talking it through may help to consolidate the incident into your memory where it belongs, rather than constantly re-experiencing it as if in the continuous present.
Visit your GP. Explain what has happened to you, and how you feel.
Your employer should be offering support, through an employee assistance programme or occupational health (OH) service.
There is no 'one-size-fits-all' formula to support you after a traumatic event. The support your employer offers needs to be flexible enough to understand that individuals will cope in very different ways.