Transmission problems

Daughter was driving car, came to a stop sign, stopped and then tried to take off and the car wouldn't move. Got it home, parked it over night got in next day to move it into garage to work on it, it moved under its own power. While pulling it into garage it stopped moving on its own. Checked fluid, was at full level. Never had any issues before. Following day got back in and the car moved about 500 feet on its own and stopped again. Thanks for reading.