Usually the lock actuator will not release; The shifter lock is controlled by the signal from the brake light switch. If it a floor shifter, first ensure the brake lights are working, that being said I have had bad switches with the brake lights working. Sometimes things gather around the solenoid, at the bottom of the shifter where the solenoid is mounted, causing the solenoid not to release. Some people report that spraying it with wd40 or similar product loosens the actuator allowing it to work normally.
It's difficult to say the exact cause. I'd make sure the 3rd center brake light is working properly, sometimes the brake switch will cause this problem. It could possibly be the shifter mechanism itself. We had one recently that the customer spilled a soda and it was freezing in there causing the shifter not to move. We would be glad to check it out for you, rough estimate on diagnosis would be 30-60 dollars and we may be able to repair it at the same time. Call us to schedule an appointment. 563 388 9363 thanks, Dale's