Reverse/No Forward

904 trans I traded for couple years ago. Itis supposed to be full race setup with Cheetah MVB. Been a running driving car.

Pulled into a parking lot and put it in 2nd. Made a bang, and it won’t go forward in any gear but reverse is fine.

Pulled the pan and it looks fine. Fluid is clean and no pieces are present.

Got the Tom Hand book out and its pointing to an internal pressure leak.

Bummer Mopar Day is Sunday and my daughter run this car. Thoughts? And thanks ahead of time