What happened to my belt?

I was driving down the road at about 40 and it
became really hard to steer like I ran out of gas but
it was on. The screen above the radio said
charging failure and luckily I was next to my house,
so I pull in and pop the hood and when I look inside
I see that the *serpentine?* Belt was completely
off but wasn't shredded but a little strip about 6
inches long or so and the pulley in the bottom back
is really loose. I want to know what may have
happened and how to fix it, thank you.