What causes the change in the Loisels' financial situation in "The Necklace"?

The change in the Loisels' financial situation is caused when Madame Loisel borrows what she thinks is a very costly diamond necklace from a wealthy friend for a party. She then loses it. It takes every penny the Loisels have to finance the replacement necklace, which costs 36,000 francs. Monsieur Loisel has to take out a series of loans, all of which must be repaid.
They tighten their belts, give up all their small luxuries, get rid of their maid, move to a shabbier garret apartment, and Madame Loisel turns into a shrew, bargaining and haggling with grocers and merchants over every half penny of expense. She also learns the hardship of having to do all the heavy housework and scrubbing herself while her husband moonlights to earn extra cash. This impoverished lifestyle continues for a decade.
Ironically, Madame Loisel finds out after all their sacrifice that the diamond necklace was simply a cheap fake.