Problem with my thermostat? CAR?

[Javier]

I have a Mustang 2002 and Today i was driving it in the city for about 15min and I went to a store for about 5 min and after I turned my car back on and I my SERVICE ENGINE LIGHT turned on I and started driving again and my thermostat was all the way to cold, and wasnt even reaching the C. It was as if I had my car turned off. So i pulled over and I turned off my car and then back on and the thermostat was back to normal "in the middle" and right before i was about to stop at my house, I was driving at about 10mph, the thermostat all of a sudden went from being in the middle of C and H to the left of the C, like if my car was turned off.

ANY ONE HAVE AN IDEA OF WHAT IT COULD BE and how to fix it? Thank you.
WHEN I TRIED TAKING MY KEY OF THE IGNITION IT WAS KIND OF STUCK OR IDK WHAT HAPPENED THERE.