There's a few painful seconds when nothing happens, but just as Anna's about to knock again, she hears the lock turn.
Elsa's sitting on the flimsy bench in the changing room, arms wrapped around herself in an attempt at comfort. Anna has no idea why she's crying, but that doesn't really matter right now. Elsa gives another sob before she manages to look up at Anna. Neither move for a moment, until her hands twitch and move from her hug, falling open slightly.
Anna doesn't even hesitate when she takes Elsa in her arms.