1995 Dodge Neon speedometer not working

We recently rebuilt my 5 speed dodge neon and pretty much replaced everything on it. At first the speedometer worked fine then it all of a sudden stopped working but some times when I would shift gears it would go up a little n thats it. When we wnt to replace the vss the plastic it was attached to fell down into the transmission and it took hours to get back up to attach a different one n get it in place. This did not work the speedometer still didnt work. My fiance said it could be a worm gear but i'm not really sure wheat that is but I had seen on another one of your answers that it could be a broken gear or slipped the shaft but it seems theres no way to really get into the transmission there what do you have to do to find out exactly what is wrong with it and what it involved in fixing it.