As the heading suggest, people who drives an automatic car (AMT car) might have faced the issue where while driving in the drive mode, the gear automatically shifts to neutral mode and the car comes to a halt. As a result, when is it taken to for repairing in the service centre, they generally suggest that there is an issue with the gear box and it needs to be replaced. The same incident happened with one of my friends who drives a Maruti Wagon R Automatic. So, the solution he followed might be helpful in your case also and atleast you can give a try before bearing expenses of changing the entire gear box.
So, my friend faced the issue two times and the solution found out after a lot of research are as follows:
1) The Hydraulic Pump Relay Circuit may have been damaged which needs to be changed. It cost around Rs. 6000/- . The picture shown below depicts the issue when the testing was done at the service centre for determining the issue. If this issue is identified by the service centre, try changing this part only for resolving the issue.
2) Rat might have chewed out and disrupted the circuit wire which can be rewired to resolve the issue. The youtube vide link : https://www.youtube.com/watch?v=gCdqYNH3e_s speaks about this issue and its solution (Credit: Technical Auto Tech)
If this issue gets resolved by the above two solutions, then well and good otherwise the instructions of the service centre have to be followed.