Nissan Micra Auto 2001 gearbox?

I've never had problems with the car until yesterday, I stopped at a junction so I went into neutral, when I went back into drive, it would not go forward!, And now, I can only go reverse. RAC came out and said it's a gearbox problem, need to have another one fitted! £800!!! I don;t have that lying around so was wondering whether it does sound like a gearbox box problem! Thanks