Bathroom sink won't drain
That doesn't make sense, but I guess it doesn't have to make sense to be a problem.  If you're sure that you didn't drop anything down the drain, I'd try a plunger.  The way to use it is not to ram it down hard, trying to power any obstruction through.  Rather make many small strokes up and down to break the obstruction up with oscillations.  If you think you did drop something in the drain take the trap apart and get it out.