engine hot A/C off light comes on?

I had a overheating engine. I recently changed my radiator fan assembly kit. And now my the needle for the engine temperature is completely cold. Also, the check engine like came on. I looked online believe that it is the thermostat that needs to be changed. I never got the chance to change it. On my way home from work the needle went back to normal the engine hot A/C off signal was no longer there, but the check engine light was still on. What do I do?