what is the term used to describe a situation where a hazard has happened and it affected humans?

The standard term for a situation where a hazard has actually occurred and harmed people is usually “disaster” or, in safety/engineering language, “harmful event” or “hazardous event.”

Quick Scoop

When a hazard is no longer just a possibility and has already affected humans, different fields use slightly different terms:

• Disaster
  • Common in disaster risk reduction and emergency management.
  • Used when a hazardous event causes serious disruption, injury, loss of life, or major damage to communities and the environment.
• Hazardous event / harmful event
  • Common in occupational health and safety, engineering, and standards like ISO 14971.
  • Describes an event or situation where people, property, or the environment are exposed to a hazard and harm actually occurs (e.g., injuries).
• Incident / accident
  • Everyday and workplace safety language.
  • “Incident” is often any unplanned event that causes or could cause harm;
  • “Accident” is more specifically an unintended event that results in injury, illness, damage, or loss.

If you are talking about community-scale impacts (e.g., earthquake, flood, industrial explosion affecting a town), “disaster” is usually the most accurate single term. For a technical or workplace context with injuries, “hazardous event” or “incident causing harm” are often preferred.

Tiny example

• A chemical stored in a factory is a hazard.
• A leak that exposes workers, causing burns or poisoning, is a hazardous event or incident.
• If that leak spreads to the surrounding community and overwhelms local response, it becomes a disaster.

TL;DR:
For general use, call it a disaster when a hazard has happened and significantly affected humans; in technical safety language, you can also say hazardous event or harmful event when you focus on the specific occurrence and resulting harm.