1998 Buick Century Driving Stopped Light Cause Red Turned Green Hit Gas Nothing

i was driving and stopped at a light cause it was red. when the light turned green i hit the gas and nothing happened. the car just drifted. i can manually shift the car and go into reverse. checked the trans fluid with car in park and running it looked fine. changed the filter and put new fluid in and also replaced the transmission modulator. still nothing. what could it be???????