My 1996 Dodge Ram 1500 with 5.9Ltr stopped going into overdrive today.?

Today on the way to work as soon as i got up to 60mph i realized it didnt shift into overdrive.. i tried pushing the O/D off on a few times hoping it would go in, but it never did. It was about 10 below out today, but i drive 20 miles to work and the o/d never started to work. what should i try to fix it??