1994 Saturn SL2 Will not shift gear

I believe this is a transmission problem but I am not sure. Your help is greatly appreciated. My wife borrowed her sisters car to take a trip. There have been no known problems with the car. Her sister drives it daily with no problems or hard shifting. (Automatic) After driving aprox 135 miles at 70mph the car began to loose speed and would not shift into another gear, only rev the rpms. The car would drive slowly to remove from traffic but estimated it would not go over 20 mph.