Why do you think Mme Forestier never contacted Mme Loisel after the return of her necklace in "The Necklace"?

I believe Madame Loisel was a very proud woman who never wanted Madame Forestier to know she had lost her necklace. To Madame Loisel, it was a shameful secret. Further, Madame Loisel, who placed such importance on money and outward appearances, knew that the loan for replacing the necklace would make her and her husband poor and unattractive. She would have been mortified by the idea of the wealthy Madame Forestier witnessing her degradation and being in a position to pity, scorn, or pass judgment on her for her carelessness in losing the necklace and her subsequent poverty. From Madame Loisel, it was better to sink into obscurity than be humiliated in front of her friend.
For the reader, of course, this decision is frustrating. If Madame Loisel had only talked to Madame Forestier right after she lost the necklace, and admitted she had lost it, she could have been spared decades of misery. Madame Loisel's own pride and over-valuing of shallow, superficial items like pretty necklaces led to her undoing.