If Mathilde Loisel did not lose the necklace nothing would have changed for her. She would have gone on being poor and miserable, longing for more.
Mathilde feels like she should be rich, even though she was born into a poor family and married a poor man.
She had no gowns, no jewels, nothing. And she loved nothing but that. She felt made for that. She would have liked so much to please, to be envied, to be charming, to be sought after. (p. 1)
Mathilde thinks of nothing but the material. Although she has a loving husband and a comfortable life, she is not satisfied. She wants more. It is because she wants more that she borrows the necklace from her friend to go to the ball.
Mathilde has a good time at the ball, but like Cinderella, she has to go back to reality sometime. When she leaves the ball, she symbolically returns to the life of poverty she has always known.
He threw over her shoulders the wraps he had brought, the modest wraps of common life, the poverty of which contrasted with the elegance of the ball dress. (p. 4)
If she had not lost the necklace, she would have returned it and continued in her relatively comfortable life. She would have remained beautiful for years afterward. She might even have found out that the jewel was a fake when she returned it. Yet because she lost the necklace, she lost everything else too. Her husband, her beauty, and her friendship all diminished from her greed.