The best avenue may be to contact the conciliateur, an impartial service which acts as a mediator between a Cpam and a user (every Cpam has such a mediator).
You should first have made a written complaint to the Cpam. If you are not satisfied with the response – or do not obtain one – you may apply to the conciliateur at the address of your Cpam addressing the letter to M. le conciliateur.
It may also be possible to do this by email or by speaking to someone on the phone or at a face-to-face meeting; your Cpam should be able to advise about this.
You should receive a letter of acknowledgement and the service may advise on other possible avenues and deadlines.
You can also find more information including a model letter by visiting ameli.fr and clicking Droits et démarches > Réclamation, conciliation...