Transmission problems!! 2002 Ford Mustang v6, auto?

I have been working on my girlfriends mustang for quite awhile now(2002 v6, auto). And Just today she went to leave her house when the car wouldn't allow her to select drive. (I just recently filled up her trans with the correct fluid and to the recommended level.) so I went to check it out and found that the button on the shifter would not go in while the car was started, off, or in "acc". but it would allow me to select the gears while the the key was in the "on" position and can however start the car while in the" on" position and in neutral. i can select any gear except park! Once I do that the shifter and button would not allow me to move it again... So if there is any advice or theorys on what is happening and how to fix it It would be greatly appreciated!! Thanx

it seems like something in the actual shifter is broken. Take it apart and inspect it. Replace it with one from the junkyard or an aftermarket 1 from autozone

I believe that you also have to depress the brake to engage the shifter. Make sure that the brake pedal switch is working correctly. The ignition switch or the internal mechanism could also be going bad.