Assuming you are referring to a vehicle shifter becoming stuck and not being able to be moved out of park, there are a few things that can cause this. The first thing to check is the brake light switch. This switch needs to be working properly in order for the shifter to be released from park. If the brake light switch is defective, it will need to be replaced in order for the shifter to work again. Another possible cause is a problem with the shift interlock solenoid. This solenoid is responsible for engaging and disengaging the shift lock when the brake pedal is depressed or released. If this solenoid is defective, it will need to be replaced as well.