It's the auto transmission manual selector switch down on the side of the transmission ..easy fix ..there's a u tube video walking you through it step by step. Part was $50at oriellys...mine just did similar ...was putting it in park and stuck in reverse won't let me shift or start it ..switch fixes issue..takes about hour to replace with common tools94,440 miles (bought new in 2003).
But just today my automatic shifter slipped out of gear while trying to reverse and it would not go back into any gear. I was able to put it back at the park position, but the transmission stayed in drive. So I took the key out, then my shifter locked in the park position, and would not unlock again (even with both the ignition switch turned to the unlock/on position and the brake pedal depressed). Now I can't start the car because the transmission is in drive, but the shifter is locked in park. I don't know......Murphy's Law I guess????