My car will start and run but wont move, why? 2000 Ford Mustang?

I have a 2000 Ford Mustang. It's been sitting in the drive way for a while. Today i crank to the car to take it around the block and its starts and runs but wont move. I can change gears and when i put i reverse it did easy back, but that's probably because we are on a hill. Please help.