I believe you are referring to 'The Necklace' by Guy de Maupassant, please let me know if I'm mistaken.
The answer is that after they realize the necklace is missing they decide to buy a new one for Mme. Forestier
In page 6/10 of the story there's a part where after looking everywhere for it, Loisel proposes that they must replace it. Due to the mishap she came to know the drudgery of housework and her husband had to work day and night, they both aged in a not so beautiful way and all the way to the end of the story she keeps wondering what would have happened if the necklace had not been lost.