Why did my 2007 chevy hhr check engine light come on?

There is no way to reset the check engine light on the 2007 Chevy HHR. It comes on when there is something wrong with the car, but it can be caused by just about anything. Due to this, it is not worth Your time hunting down a random issue unless it is something very obvious that has gone wrong. The best thing to do is take the car to a mechanic and let them scan it with an OBD tool. This will allow them to read the trouble code and tell You where the problem is. Once You know that You can go about getting it fixed, which will turn off the light. One thing to check however is the gas cap and fuel door. If the cap is missing or the door is open then the light will come on. This is a silly thing to go to the mechanic over, so make sure You check it first.