Prior to the loss of the necklace, Madame Loisel is famously described at the very beginning of the story as
...one of those pretty and charming young creatures who sometimes are born, as if by a slip of fate, into a family of clerks.
Although she is pretty enough, and should be thankful to have a decent husband who keeps her at home with a young, peasant Breton girl for a maid, Mathilde is quite unhappy. She isn't satisfied with any of her surroundings. She has huge dreams of fancy and elaborate ceremonies, dinners, tea parties, and the likes of that.
Not only is she sad, but also ungrateful. When her husband finally manages to get them to a ball sponsored by his workplace in the Ministry of Education, she is sad that she does not have a new dress, or jewels, to go.
Here is when her husband proposes the borrowing of the necklace. When she gets her hands in that huge, flamboyant and shiny piece of pure extravagance, she realizes that this is it for her. At the party, she is ravishing, and wants to make sure she shines as brightly as her borrowed necklace. All the excitement is over after the ball ends, and she realizes that the necklace is gone.
After the loss of the necklace
Since the Loisels decide to hide the fact that the necklace is lost, they had to make up for it in a short period of time. After they found the replica to purchase it, they had to sacrifice their entire lifestyle to be able to afford paying for it.
Now, Madame had every reason to complain. This is the first time she came to know what it is to really sacrifice and what true problems really look like. If she was ever angry and disillusioned about her life, things were just about to get worse. We know this in the way that she changes physically, socially, and even behaviorally, after so many years working cleaning floors, moving to a smaller place, haggling the prices of things, and having to live like the lowest peasant she could have ever imagined to become.
Madame Loisel looked old now. She had become the woman of impoverished households--strong and hard and rough. With frowsy hair, skirts askew and red hands, she talked loud while washing the floor with great swishes of water.
She was no longer cute; no longer dainty, nor delicate. She was "loud", and rough, having lost the flower of femininity that is so unique to a fragile woman of the household. She even had the courage of showing herself up to Madame Forestier, who was less than pleased to have seen her, but was equally shocked at the state of her friend's appearance.
Perhaps this was not too bad for Mathilde. After all, she did learn to appreciate the value of money, in the end. Her vanity is presumably gone and certainly there are no more dreams of exquisiteness ruling her day. We never get to know what is her reaction when she finds out that the necklace was fake, but it is quite easy to imagine that her shock was quite intense.