Why won't my car start?

Why won't my car start? I have a 2000 Ford Expedition 5.4 liter engine. I currently live in Wisconsin, and the winters have been hard on my battery. Since it started getting cold this year, I’ve been having trouble getting The Beast to start. Up until a week ago, a simple jump would do the trick. Then, last week, I went to start it and it was giving me the usual trying to turn over but slowly sound that indicates that I would need to jump it.