Jack up the car, crawl underneath, see if there is a cable or rod between the transmission and the throttle linkage. It should be connected to a small lever on the driver side of the transmission, next to the shift linkage. It may have come loose or it needs to be adjusted. It is not difficult to adjust...lengthen or shorten the linkage and then road test to check for the kick down.
I guess I should ask...did your car downshift before?