transmission, 1994 Hyundia Elantra 1.8 auto?

My mom went to the bank placed the car in park did her deeds then went to place the car in drive she heard a pop then it didn't move. She told me she only heard it rev up saw it rpm's go up but didn't roll any where. Before then the car did not over heat or slip at all, I also check the pin to see if maybe the pin slip out,negative. Changed the trans.filter to eliminate clogging,negative. I'm really trying to prevent from having to buy a new trans. please help...