Transmission won't shift

I've got a 92 Ford Tempo with automatic transmission. After a particularly cold night, I warmed the car as normal but discovered that when I put the shifter into reverse that the car did not shift out of park. It also will not shift into drive as well. I have checked the fluid levels for the transmission but it shows full when the car is fully warm. I've only got rudimentary automotive repair skills (computer tech by trade), and so aren't sure where to go from here to pin down the issue. Any suggestions/solutions would be most helpful.