Guy de Maupassant does not specify the exact place and time of loss. Mathilde Loisel, the charming young wife of a clerk, borrows an expensive diamond necklace from her wealthy friend Joanne Forestier. She does it to fit in with the wealthy people she would meet at the ball. As a result, she gains the anticipated attention from men and high society. She enjoys the evening but eventually loses the accessory.
Detailed answer:
Theoretically, it could have happened at the ball or during the trip home with her husband. When Mathilde realizes that the necklace is gone, she recalls it was on her until they left the ministry. If it fell during the walk, at least one of them would have probably noticed it. Thus, Madame Loisel most likely dropped the necklace in the cab. The noise from the trip would have masked the sound of it falling. Monsieur Loisel tries his best to find it on the streets, in the police, cab companies, and newspaper offices. His efforts were a waste of time as there was no lead.
The pair decides to replace the necklace with a similar one. It results in them going into debt and ruins their life for the next ten years. This hardship aged Loisel and ruined all her hopes for a wealthy lifestyle. When Mathilde meets Madame Forestier, she finds out that the diamonds were fake and cheap. Hence, the young woman simultaneously lost the necklace, ten years of her life, and a chance for a luxurious life. All of this happened because she decided not to tell her friend about the loss. This symbolic place and time highlight the value of honesty. Mathilde could have admitted that she lost the necklace and saved her life, but her pride was greater than being truthful.