If the target is already tapped at the time Plunge into Winter resolves, you'll still scry 1 and draw a card.

You can cast Plunge into Winter without a target just to scry 1 and draw a card. However, if you do choose a target, and that target is illegal at the time Plunge into Winter tries to resolve, the spell won't resolve and none of its effects will happen. You won't scry, and you won't draw a card.