Okay...How do I fix it..Or does a shop have to.

I have a 1990 Grand Prix Turbo (actually my wife does) and my truck is in the shop so I borrowed her car to go to work. On the way home I drove a little fast but wasn't "dogging" it. As I turned on to my road, I downshifted (went from Overdrive to drive) and when I did that it felt like I had pushed the brake. Then I tried every other forward gear, and it did the same thing. I pulled over and put it in park and killed the motor. After a minute or so, I restarted the car and none of the gears would work except neutral. What I mean by that is in every other gear the car acts like it's in park even if you try to push it it won't roll (except in neutral). Since my wife is pissed and I know nothing about Grand Prix's, I am hoping one of you guys could help me out...

Thanks