Why was Matilda sad after the ball?

The night of the ball had been the realisation of a long-cherished dream for Matilda, when she was the focus of all the attention and every eye admired her. But the happiness was short-lived and degenerated into a shock of the worst nightmare when she discovered that the necklace was missing from her attire. Her mind was ridden with forebodings of how she would face Madame Forestier and pay up for the loss.