1. You may have engaged the anti-theft rod or steering wheel lock in the steering column. Generally this happens when you turn your car off with the wheels cut hard in either direction. You can generally remedy this problem by "jiggling" the steering wheel right and left while attempting to turn the key. 2. The key is bent or otherwise damaged. Take a good look at your key to insure that it is not bent, cracked, chiped, or damaged in any way. If it is, try starting the car with an alternate key. (You do have more than one key for your car right?) In a pinch, you may also be able to repair a slightly bent key by squeezing it flat in a vice. 3. The ignition lock cylinder is shot. Over time ignition locks can wear out. You will note a bad lock cylinder because the lock will be difficult to turn with any key and often will not turn at all. As a quick remedy, you can try a bit of penetrating oil (WD-40 or similar) inside the cylinder and on the key, but this is likely only a temporary solution. At some point you will need to replace the cylinder. (The sooner the better.)