MY 2005 CTS is a daily driver, wife drove it Sat. AM, I was going to drive it Sat. PM, & would not start. Turn key & lights come on, turn to start, there is one click under the hood, & motor does not turn over. Tried it with headlights on, same thing & lights do not dim like a bad battery. I have a key that was made at local store that does not have the chip in it to unlock doors, of course it will not start car, but I tried it anyway, same symptom, not I do not hear the "click" under the hood. That lets me know that the its probably not the key,,,

 

You have to start with the basics.
Have the battery tested. The old headlight trick can be very misleading.