I could not get the car to go into any gear

Hello, driving on the highway seventy five miles per hour, all the sudden I started to slow down I went to the shoulder I could not get the car to go into any gear. No noises, no shutters, clunks or squeaks. Had no shifting problems up to this point. I had a radiator leak for a while prior but that was repaired. It made a terrible grinding noise in the rear of the car in reverse. Other drivers say they heard it all the time. I only drove it once the day before it went out. It has been sitting since December I think it is the torque converter but not sure. Is there test I can do to know before I drop this transmission?