If possible, take it back to the boutique or department store and they will repair it. If it is not on a leather portion or is in some other fabrication, you may have to take it to a shoe cobbler to reglue/attach it.
I was told by two managers, my home location in Tysons VA and in Palm Beach FL that if something happens with cloth or fabrications other than leather that they will inspect you shoe and if it "interferes or can not be restored back to the Chanel integrity standards" they will not repair it and encourage you to get it repaired elsehwere. Also, there are issues of wear and tear when it comes to shoes that is taken into consideration (i.e., frequent/infrequent wear and terrains etc...).
If this has occurred within the first year I would take it back for a repair. If my memory serves me correctly, you do have to pay for some repairs depending on the situation, unlike purses. I have only had repairs sent back through the boutique on some of my flats that were over 20 years old---bow tie broke off. I think????, not sure, the department store may be more lenient and accomodating on occassions. HTH