What causes the key not to turn?

Hello,

Just to clarify, you cannot get the ignition key to turn or you cannot get the key out of the ignition?
If you can put the key in but not be able to turn it, then you could have a stuck lock cylinder. It can also happen if the steering wheel turn all the way to either direction. The steering wheel lockout will keep the key from turning.
In this case grab the steering wheel and apply pressure in the same direction as the wheels are turned, the lock pin should release and you will able to turn the key.

You might find this article helpful:

https://www.2carpros.com/articles/ignition-switch-wont-turn-stuck

I have attached diagrams for your reference.

Please let me know of any questions.

Thank you.