1990 Mercury Sable transmission problems

Vehicle when was in park, the key was released and put back into the ignition, before consumer could get back into the car, it rolled backward and hit a tree. When hitting the brake pedal the vehicle did not stop, occurrence on level pavement, no injuries involved.

Car cruised just fine 25 miles to town, got all my stuff done in town, pulled up to a stop sign on a hill and the car would not move, engine ran just fine, the transmission would not shift into gear, car only has 110k on it, had to roll back off the road and stop, park gear worked, after an hour car still would not shift into gear, 4 days later I went out to try it again and the thing went into all gears and took off.