Do you have a second/spare key try that first, if that fails you have a problem with the tumblers in the ignition lock itself. Any automotive repair shop repair shop should be able to replace the ignition lock if needed but call your local lock smiths and explain your problem. Where I live some of our local locksmiths can do this work cheaper than we can at our shop.
I had the same problem. In my case the "tumblers" were worn. Looking into the keyway you may see a brass colored part coming up from the bottom which is likely one of the set of 9 "tumbers" and it is out of place. I was able to gently insert a thin object and pressing slightly down and pulling out put it in place such that I could use my key. Once the key is in DO NOT REMOVE IT untill you get it repaired. I bought a new lock assembly from GM and installed it. The following link will explain how http://www.sparkys-answers.com/search/label/Lock%20Cylinder
call a locksmith. it's not worth trying it yourself...trust me!! I called my local auto shop to see if I could have it towed in and they suggested that I call a locksmith. He came and fixed it in about 45 minutes. He was amazing and it only ran me $100! Wish I would have just called him first before I started trying to fix it myself!