Clogged Drain!!! What Can I Do? Not Working!?

so my sister and I share a bathroom when I come home. While I was at college, apparently she didn't clean it ever. So now that I'm back with them for the summer, I absolutely can not stand it anymore. I HAVE to clean it. Plus, it's stopped draining!!! At all! She would use it and let the puddle drain over night! Are you kidding me? So!!! I just tried baking soda and white vinegar to see if I could blast or bubble it through. I also tried some hydrogen peroxide although I know you're not supposed to do that... I just can't stand it. What else can I do? I got the whole sink spotless and shining but the drain still wont drain and I'm sure there's crazy mold in there. What can I do??? I just can't leave it like this!