Will not go into forward

Grandchild trying to help out my eighty year old Grandpa with no BMW dealership nearby. Any help we could get would be very appreciated!

My grandpa was recently driving up a hill and his engine stopped working and he threw two belts off. He now only has reverse and he cannot drive forward. It shifts currently into all gears. We replaced the two belts, the car runs fine. However, it will not go forward. Does anyone know if there is a reset or some electronic thing that needs to be reset or if it's a transmission failure or if there might be some other explanation for what's going on? We do not know if the belts caused the problem or if the problem solved the belt failures.

Thank you!