No reverse 904 trans

1973 Dart slant six 904 auto. I came home from work tonight and went to back the car in to my driveway, put car in park to open the gate and when I put the car in reverse there was a loud clunk and and no reverse. All other gears seem to be working fine as I had to drive the car around the block to line up for a straight shot to park in front of my house. I did not lift the hood as it is dark and raining tonight. Any idea as to what might have happened? valve body issue or pull trans? I have another trans but do not know the condition and I think it will get rebuilt before I install it if I need to. the trans has been working fine up untill tonight. Looks like I'll be pulling the 74 Challenger out of the garage to drive in this rainy weather.
Thanks in advance for any advice as to what it might be,
Kevin