First of all, don't drink it if it smells spoiled. You'll know if it's spoiled even if you've never smelled it before. It's very obviously different from "good" milk. Also, milk tends to curdle when it goes bad. if it's lumpy, don't drink it.
Milk is pasteurized these days, which means they kill all the bad organisms in the milk before it gets to the store for you to buy.
Milk will still, inevitably, become re-infected at some point. Depending on what organism is in your milk by the time you drink it "spoiled", you'll likely come down with some kind of food poisning with effects ranging from nausea to cramps to diarrhea, etc, etc. If that happens, you probably want to see a doctor.
You could be sick with Salmonella, E. Coli and others. You might not feel sick for hours or days, depending on how badly your Milk was overrun with baddies.