How does madame loisel lose the necklace?

When Mathilde finds out that she has lost the diamond necklace of Madame Forestier, she is in a depression. Mathilde and Monsieur Loisel then have to work for 10 years to come up with the 36,000 francs to buy a new necklace. She works as a maid to save the money and her husband does work two jobs..