1991 Mercedes Benz 300se what wrong with my car

Transmission problem
1991 Mercedes Benz 300se 6 cyl Four Wheel Drive Automatic

I own a 1991 mercedes benz 300 sel and I was driving I heard a big boom smoke came out from the back of the car, then it stopped and the car would not move I looked at the back tires where the noise came from and the tires look sinked slightly inward and the tow truck guy told me the transmission look like it fell now the car wont move why would this happend and what could this be? And if it is the transmission what is the approximate price for the transmission to be replaced and if the tires are sinked in did it damage my car totally?