Hello, welcome and thanks for asking your question. My name is*****'m a Certified Auto Technician with 29+ years experience. I will do my best to assist you. please keep in mind that I may need to ask questions of you before I can answer. When you reply please feel free to add as much detailed information as possible pertaining to your concern.
This is normally caused by a faulty Ignition Lock Cylinder. Sometimes the tumblers bind up causing the Lock Cylinder to get stuck. Sometimes you can temporarily fix this by spraying some graphite or lock spray into the key opening. This may free up what is causing the binding and get it going again.
Another thing that can cause this concern is if the steering wheel is locked. If this is the case, turning the steering wheel slightly to the right or left will release pressure on the locking mechanism and allow the key to turn.
If neither of these recommendations help, you will have no choice but to have the Ignition Lock Cylinder replaced. Just be sure to code it to the ORIGINAL keys so that you don't have to program new keys....
If you want to do this yourself, let me know, I should be able to come up with some info that will help you get the lock cylinder coded properly and replaced.
Keep me posted and let me know if you have any additional questions...