Why was Matilda sad after the ball?

The night of the ball was a realization of a long cherished dream for Matilda, when she caught everyone's attention. She had borrowed a beautiful necklace from her friend. As she was looking good, everyone admired her. But the happiness was short-lived and turned into her worst nightmare when she discovered that the necklace was missing from her attire. She was tense and wondered how she would face Madame Forestier and pay up 1 the loss.