the check engine light is on because the car's computer has detected a system that is operating outside of the normal range.When this happens, the light comes on to alert you to get it checked for the diagnostic code that caused it. It can be anything from a loose gas cap to a sensor going bad.
What you should do is get the code pulled from the computer. A local auto parts store (like an Auto Zone) can do it for free. Write down what the code is and let me know what it is. This will help greatly in diagnosing what turned the light on in the first place