Villageblue

Hi all, Having a problem with my auto gearbox. Started the car as normal moved the gearknob to drive, drove for 100 yrds and stopped tried to put the car into park and it wont let me. It's as if there is a block on the gear knob between the reverse and park parts of the shift. Am putting my foot on the break. the car is 4 yrs old. any ideas?