Transmission Issues...

I have a 94 Mustang with a 3.8 and an Auto...tranny.. A week or so ago I was cruisin along and when it shifted from 2nd to third My girl jerked and I heard a huge Thud... Almost like I ran someone over and from there on it wouldnt engage in any gear... Tonite I started it up just to hear it run again and put it in first and it rolled.. I put it in reverse and it rolled again.. A few times later its back to not engaging in any gear. Any idea whats going on? ANy help would be awesome.

any ideas??

My auto teacher said it might have been just low fluid but fluids ok...Does it need to be rebuilt?