My radiator light went on. Then my heater started blowing cold air. Then my car overheated. Why?

Might Be The Thermastate Have You Checked The Coolant Level?

You might need the thermostat replaced.
Did you see any water under the hood or under the car?
If there was water under the hood you could need new hose(s)
If there was water under the radiator you could need a new radiator.
Or you could just be low on water/antifreeze.

Check thermostat.. needs replacement.

My radiator light went on. Then my heater started blowing cold air. Then my car overheated. Why?
You may have a cooling system leak. If you have been losing coolant, you should get this diagnosed before it turn into something major.