When a toyota prius abs warning light comes on, what has happened?

If the ABS light has turned on at Your dash in Your Toyota Prius that means that there was a problem that was detected with the antilock brake system in Your car. Usually You can use an OBD II scan tool to read the code that was saved to the ECU that will tell You what fault was detected. I recommend that You address any problems with the ABS system as soon as possible. This system helps Your vehicle stop and is crucial to the safety of Your car. Usually a dealership can help diagnose and repair any problems that have come up with the system.