This was a sudden failure so I suspect it is not due to loss of auto transmission oil. There would be a pool of it under the car.
Some years a go I had a similar failure in a Mark 1 Triumph 2000 Auto. There was a metal disk (the drive plate) that bolted to the end of the crankshaft. The torque converter then bolted to it. The engine tore the centre out of the drive plate with a horrible grinding noise, resulting in the symptoms you describe.
Car technology has moved on since 1969, so things may be different now. But thanks for reminding me of that wonderful old car. It cost me �6 for a drive plate from a scrap yard, which I fitted myself - those were the days. If your problem is similar and there is no other damage then I would guess it is going to cost around �300-�400.
Let us know how you get on.