Transmission shifts into reverse but no throttle.

As stated. You can feel the transmission shifting into reverse but absolutely zero throttle response, you can press the pedal all the way to the floor but nothing happens, the engines RPMs do no change or anything. I recently asked a transmission mechanic about it and he said that sometimes when the battery dies or is replaced in these vehicles it goes into a sort of limp/anti theft mode. How can I diagnose the problem?