In "The Necklace," how did Mathilde's life change after losing the necklace?

Mathilde's life took a change for the worst after the loss of the necklace. Namely, because instead of swallowing her pride, owning to this fact and confess to her friend, she decides to take matters into her own hands. She goes out of her way to replace the lost necklace and, as a result, went in complete financial ruin. 

Madame Loisel came to know the ghastly life of abject poverty. From the very first she played her part heroically. This fearful debt must be paid off. She would pay it. The servant was dismissed. They changed their flat; they took a garret under the roof.

Both, Madame and Monsieur Loisel had to give up their lifestyle to replace this piece of jewelry. Monsieur went on to invest his inheritance, and give up all this belongings. Mathilde had to take jobs and work harder than she ever had. The hard work and sacrifice reflected in her face and her daily actions. 

Madame Loisel looked old now. She had become like all the other strong, hard, coarse women of poor households. Her hair was badly done, her skirts were awry, her hands were red. She spoke in a shrill voice, and the water slopped all over the floor when she scrubbed it.

The result was that she grew coarse, bitter, and overwhelmed with these things. Her beauty disappeared, and she became essentially an angry and ugly person. We know that all of this was totally unnecessary. The jewels she had borrowed were fake in the first place and there was nothing to be in need of replacement. The shock comes to the reader, and we never get to learn Mathilde's reaction as the ending comes to an abrupt stop when the truth is revealed. 

What unexpected outcome occurs in Mathilde Loisel's life after borrowing a necklace in "The Necklace"?

In "The Necklace," Mathilde Loisel and her husband toil for ten years to pay off a sixteen-thousand-dollar debt. They incurred this debt when they purchased a replacement for a borrowed diamond necklace that Mathilde lost at a formal engagement. She wore this ornate piece of jewelry in order to appear more glamorous and wealthy than she was in reality. Losing this necklace, however, was a major turning point in her life. As a result of this event, she developed from a vain, spoiled, and insecure girl into a mature woman comfortable with her true identity. She learned to work hard and take pride in herself.

Before borrowing the fateful necklace, Mathilde considered herself a victim of circumstances. Married to a government clerk, she felt that she deserved a more luxurious life. She did not appreciate her comfortable home and life. After losing the necklace and plunging the couple into debt, "Madame Loisel came to know the ghastly life of abject poverty."

Yet instead of acting helpless and consigned to her fate, she takes action to help herself and her husband:

From the very first she played her part heroically. This fearful debt must be paid off. She would pay it. The servant was dismissed. They changed their flat; they took a garret under the roof.

Earlier, Mathilde merely complained to her husband about whatever she lacked (like tapestries, fine furniture and food, fancy clothes, footmen, etc.) without thinking about how to solve any problems herself. Now, she actually pitches in and works to repay the debt. Previously, she lived under the illusion that she was deprived; she now learns what life is like without a large furnished apartment and someone to wait on her.

Also, Mathilde originally was a housewife who did not work or have any children to care for. After the necklace debacle, however,

she came to know the heavy work of the house, the hateful duties of the kitchen. She washed the plates, wearing out her pink nails on the coarse pottery and the bottoms of pans. She washed the dirty linen, the shirts and dish- cloths, and hung them out to dry on a string; every morning she took the dustbin down into the street and carried up the water, stopping on each landing to get her breath. And, clad like a poor woman, she went to the fruiterer, to the grocer, to the butcher, a basket on her arm, haggling, insulted, fighting for every wretched halfpenny of her money.

Before the necklace debacle, she led an idle, easy life. Instead of exerting herself with any physical labor, she sat around and complained. Instead of "haggling" and trying to save money, she just spent her husband's hard-earned money and manipulated him into giving her more.

Physically, Mathilde changes. No longer a "pretty and charming" girl, she has

become like all the other strong, hard, coarse women of poor households. Her hair was badly done, her skirts were awry, her hands were red. She spoke in a shrill voice.

In fact, her wealthy friend who loaned her the necklace can no longer recognize Mathilde, who has aged beyond recognition.

Most significantly, Mathilde now takes pride in herself. Upon reuniting with this friend, Mathilde does not feel ashamed of the truth. She finally admits to her friend that she lost the diamond necklace. Mathilde expresses pride in successfully working off the debt. She declares,

I brought you another one just like it. And for the last ten years we have been paying for it. You realize it wasn't easy for us; we had no money .... Well, it's paid for at last, and I'm glad indeed.

Then she smiles with pride and happiness. The outcome of her losing the necklace is her gaining self-respect. Mathilde finally accepts who she is without trying to aspire to or pretend to be someone she is not. She no longer just covets what she does not have; she finds satisfaction in hard work, self-sufficiency, and honesty.

After paying off her debt in "The Necklace," Mathilde wonders what her life would have been like if she had not lost the necklace. The narrator does not suggest an answer to this question. What do you think would have happened to her? How does Mathilde’s character change after she loses the necklace?

After Mathilde loses the necklace, she learns what hard work really is. Prior to this, she had wasted her time, pining for a more luxurious life. Perhaps if she would have worked as hard as she does after losing the necklace, she might have been able to occasionally spoil herself with some small luxuries. Upon losing the necklace, she must work with her husband to repay the debt. There is no mention of her spending her time daydreaming about luxuries she doesn't have. She just works and perhaps truly learns what a struggle it is to work hard and still remain poor. 

If she would have found the necklace, she would have returned it to Madame Forestier. There is the possibility that the necklace would be returned and nothing more would be said. However, suppose that Madame Forestier tells Mathilde that the necklace is a fake. Mathilde might be offended. But, maybe she would then consider that it couldn't have been the necklace that changed her attitude and made people flock to her at the ball. Maybe she would realize that the change in her attitude was why she had such a good time. Recall that she is wearing a nice dress but one within their price range. And she is wearing a necklace made of paste. It is her attitude that had changed. This could or could not have changed her outlook on life. 

The other possibility is that she returns the necklace, still believing it is made of diamonds. In this case, it seems likely that she would go back to her old ways of dreaming of a more luxurious life, of dreaming of the next diamond necklace.