WHAT SHOULD I DO NEXT TO IDENTIFING THE PROBLEM?

IT HAS A 4 SPEED AUTOMATIC TRANMISSION #4L60-E.
ALL THE GEARS WORKED, THERE WAS NO SLIPING, OR ANY GRINDING NOISES. EVERYTHING SEEMED TO BE WORKING FINE. THEN I CAME TO A STOP AT A RED LIGHT. WHEN THE LIGHT TURNED, IT WAS LIKE THE DRIVE LINE JUST VANISHED - IT SIMPLY WOULD NOT MOVE. IT WOULD NOT GO FORWARD IN ANY GEAR OR GO INTO REVERSE. I HAD TO HAVE IT TOWED HOME. THE TRANS FLUID LOOKED OLD AND BROKEN DOWN OR DELUTED, BUT THERE WAS NO NOTICABLE SHAVINGS OR DEBRIS IN IT. I REPLACED THE FLUID AND CHANGED THE FILTER, BUT THE PUMP DOESN'T SEEM TO BE FUNCTIONING. A COUPLE OF MONTHS EARLIER, THE HEATER CORE WENT OUT & MY BOYFRIEND PUT SOME KIND OF ADDITIVE IN TO FIX THAT PROBLEM (HE DOESN'T REMEMBER WHAT IT WAS CALLED) - COULD THIS HAVE CAUSED THE PROBLEM I AM CURRENTLY HAVING WITH THE TRANSMISSION?

NO PRESSURE TEST WAS DONE. HOWEVER, WE CHECKED THE TRANSMISSION FLUID LEVEL BEFORE AND AFTER STARTING THE ENGINE. THERE WAS NO CHANGE IN THE LEVEL - THE FLUID DID NOT GO DOWN WHEN THE ENGINE WAS RUNNING.