I drove my car this morning. Now, an hour later, when I put the key in the
ignition it will not turn. I can't start the car.
Try removing and re-inserting the key while putting a little rotating pressure on it. Also, sometimes the steering column will lock up if there is pressure on it from leaving the wheels in a turned direction (especially on a hill). Also try adding a little lubricant in the key cylinder. Hope this works or you may need to call the a locksmith. Be careful and don't break the key in the cylinder.
I had this problem. Turns out my VCIM had to be replaced. $779 repair bill. Haven't had it happen to me since replacing the line. It has something to do with the OnStar wiring.