2002 Neon SXT Transmission problem

2002 SXT with 604-41TE 4 speed automatic. Owner that I purchased this car from broke the trans cooler line and run the trans out of fluid. He fixed the cooler line, then pulled the pan and replaced the filter and filled with fluid. It then would not move unless he would rev the engine. He drove it down the street and he heard a pop from the trans and it would not move. I pulled the pan,there was no metal shavings in it. Pulled the trans out,pulled the pump,it was ok. Pulled the input clutches assembly out, the clutches were ok they were not burnt. Anyone have any idea what might have been the loud pop that was heard. Or what might be the problem. Thanks

Did he fill it with enough fluid and what fluid did he use?

Thanks for the reply, He said he put the atf+4 in when he filled it. I checked the fluid before I drained the pan,and it was low. I drained the pan and only got about 3 quarts out. I checked the filter out it was seated ok, so put the pan back on and refiled with the correct amount of atf+4, it still would not move. I guess should of done a pressure check before I pulled it, but I just thought might be an oil or input shaft broke. Thanks

i had that problem and had to have my transmission rebuilt...