Shifter stuck in park

Are you talking about the button on the shifter knob itself? If so, that's how it is designed to work so it can't accidentally be moved to a different position.

button works. shifter wont shift from p to r

Likely an electrical problem. I believe there's a solenoid of some sort that can prevent it from shifting our of park if its not getting power. Also with mine I sometimes had to push straight down on the shifter and then it would work. I think something was broken that allowed the shifter to move up and not engage properly. Sorry, not great advice here, but maybe something you can use.

Is the shift supposed to move from neutral to drive and back with out pushing the button on a 2006 Pontiac grand prix