1997 900S Displays Temp 30F

My 16 year old son drives this and it overheated yesterday. I can't find any leaks but the reservoir was empty. With a 16 year old driving, it could have been displaying Low Coolant for some time but I don't know. I do know that after filling up the reservoir and running the car for 5 minutes, the temperature needle didn't move off cold and the display shows "Temp 30F". It's 75F outside today so is that an indication of a temperature sensor failure? I do my own work on my Toyota, Nissan, and Honda but this is the first time the Saab has had problems. Thanks.