Originally Posted by 75 racer
Sounds like the sprag clutch might of broke or the foward apply piston seal blew out. If you can get your hands on a scanner that has special functions capabilitys I would comand the transmission into 2nd and see if it moves. Either way the trans needs to come out and torn down.
The same thing happened to my Grand Prix last week. You may or may not be able to move the car in first, second, or third gear; but fourth gear (D) isn't going to work. Unfortunately, unless it'* something simple like a loose connector, you may be looking at a transmission rebuild.