1995 Grand Am Automatic won't engage drive or reverse?

- Hello, my daughter has a 1995 Pontiac Grand Am SE, Quad 4, Automatic, 51,000 miles. She was at a light, in drive, it changed and something broke and she was barely able to get to the side of the road. I had it towed home as she was close by. Engine starts and runs fine.

I'm thinking torque converter. Transmission is full, no leaks, no previous problems, she had driven over 60 miles earlier in the day. What else might it be? And how much to fix the torque converter if that is what has gone bad?

Thanks!