I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to hear that this happened.
Slapping someone is battery. It is a crime, and a civil cause of action. Assault is when a person threatens someone with an unlawful or offensive touching. You could still call the police and file a report, yes. They may question the delay in contacting them, but the statute of limitations for them to prosecute is one year after the incident, so there's plenty of time.
Your employer does not have to fire you if the evidence is inconclusive. They can't assume that your intent was malicious just because they couldn't substantiate what you said. When two co-workers have an altercation, the employer can choose to fire either, neither, or both of them. They may be worried that if they let you go, you'll sue them for allowing sexual harassment in the workplace. If this is an ongoing pattern of behavior, this could be considered a hostile work environment. If you choose to sue your manager or the employer, you can subpoena the video footage. There is no general right to view it without a court order.
If this type of thing happens frequently, by one associate or others, and your employer does nothing, then you may have a claim for a hostile work environment. However, I am hoping that this was an isolated incident and that you will not have to worry about it happening again.