Can't shift out of park

Well I bought the car with 25k miles on it and now its a 30k so me, beign on top of things, thought I would take it in to my mechanic to get the 30k service done on it. (transmission flush, coolant flush, etc) I know a lot of you will probably yell at me for paying for that but I dont know enough yet to do that, although I am trying to learn. Anyways all seemed good and I started up the car and drove off the lot and went to the gas station. I fill it up with regular unleaded gas, get back in the car, it starts fine, but I cannot shif it out of park. The button on the shifter does not allow being pressed, and I cant move it at all. I try pressing on the brake at all differnet kind of degrees it just won't shift.

So I call the mechanic from the gas station and ask him whats up why can't i shift out of park. He tells me to turn it off, turn the key to the on position, shift into neutral, then shift into drive. That works fine, but If I ever shif tinto park then I'm stuck in park, button becomse stuck again and I can't shift out of park. So I have to start the car like this every time now. He was leaving for the day so he said bring it in tommorow morning. He mentioned something about a cellunoid block or something I didnt hear him he was mumlbing. Anyways what do you think the problem is, and why would it all of a sudden happen after the service? I better not have to pay for this to be fixed now..

Any input IS MUCH appreciated so I know what the hell to say to them tommorow morning. Thanks!

2002 3.8

Yea I dont think that makes sense though because if I am shfting from neutral to drive, or any gear for that matter the button works fine. Its just locking me from going from park to any other gear.