Describe what happens when the girl with the dark hair falls on the floor.
I assume that you are talking about what happens in Part II, Chapter 1. This is the place where the dark haired girl first makes contact with Winston.
In this chapter, she is walking along at their workplace with her arm in a sling. She falls down in front of him and he helps her up. He finds that she has placed a slip of paper in his hand. All it says is "I love you." He finds this out a while later back at his cubicle. This is that start of their relationship.