The release mechanism (rod, cable, whatever) inside the tailgate must have broken or come loose. Probably a nylon clip. You'll probably have to take the panel off the bed side of the tailgate to get at it. I just looked at my '02 and there are 6 screws across the top part. The bottom is held in with plastic push pins, which you won't be able to get to with the tailgate closed, but they should pop out anyway. At least the plastic panel should flex enough to let you get inside the tailgate. Then you can pull the rod or cable on each side to get the tailgate open to fix it.
FYI - I haven't been inside a Sport Trac tailgate. I'm just going on having been inside pickup tailgates and the rear doors of Aerostars and my '94 Explorer.