92 Chevy Lumina transmission pump?

Ok at first I had a lil leak and it was coming from the gasket needin replacing, then I got lazy and just filled it up with trans fluid when needed. One day I made it to my destination with no problem but when I tried to leave the car went into gear but would not move at all. I had a cooky transmission tech look at it and they said a pump was out I have my doubts because of the fact he was talkin in circles and not answering me directly. Can n e one tell me what may have happend, it only has 135.000 miles on it and it is 15 years old, nothing was leading up to this prob as far as shifting or putting into gear. HELP please