I had made a note to address this when I flushed the transmission and had the pan off, but I forgot about it. A very common problem with the GM 4T80e is that the shift lever can get stuck in gear and the car won't be drivable. It can get stuck in "Park" or any other gear when using the shift lever. Too often it's diagnosed as a converter clutch or a blown clutch pack problem, but many times (not always) it's due to a broken small plastic retainer clip that holds the internal shift lever link rod to the rooster comb.
The pic shows how the little clip keeps the little link rod fastened to the internal shift mechanism. If it breaks or deteriorates (as it often does because it's made of plastic), the rooster comb will move due to the gear shift lever input from the passenger compartment, but it can't transfer that movement to the actual shift lever rod itself because the clip failed and the link rod has dropped out of position.
The clip functions and looks just like a door lock rod clip -- In fact you could probably use a door lock rod clip in an emergency to reattach the shifter link rod.
Bottom line, if you have a 4T type transmission (I think this also applies to other 4T models), and it won't go into "Drive", or any other gear -- or is stuck in a particular gear, then a simple check is to drop the pan and look to the right of the rooster comb to see if that little plastic retainer is still there.
