Hello and welcome to Just answer.com, Super mechanic here. About your vehicle.
That will either be the key, (you may want to try a spare key) or the lock cylinder itself. There is a separate procedure for when the cylinder will not turn to replace it.
ASE MASTER
ASE Master tech. 30+ years mechanic, shop forman, service manager
The price is not real bad on them. Just over an hour with coding the new one.