Just started to coast and now we have no forward or reverse

The most likely cause of this is either we have a leak, or they did not fill the unit enough when the topped it off.

Either way we need to start with checking the level of the fluid.

https://www.2carpros.com/articles/how-to-check-and-add-transmission-fluid

Next, we need to inspect for a leak if it is low.

Here is a guide that will help with this as I assume the leak will be with the pan gasket that was just changed:

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission

I highly doubt that this would have anything to do with changing the fluid as the mileage was not that high so I suspect we will find something with the above info.

Please run through this info and let us know what you find with this. Thanks