I agree with Bing - If you don’t know what a serpentine belt is, you shouldn’t try to replace it. Have it towed to your mechanic and have them fix it.
You don’t say what model the car is, how old or how many miles it has on it - if you’ve been having issues with the battery not charging it could be that the alternator was going and a worn bearing caused it to seize and then subsequently broke the belt, but that’s just a guess. Your mechanic can figure it out when s/he puts a new belt on.