Se você perdeu seu certificado de autenticação de cliente StartSSL, pode se registrar novamente com um ID de usuário diferente (que lhe dará um novo certificado) e contatá-lo para associar sua conta.
Na FAQ :
14.) I've lost my client authentication certificate, what shall I do?
Make sure that you are using the same computer and browser you used to register. If you are certain that you've lost the client certificate and you can't login anymore, register once again by using a different email address (if the original certificate hasn't expired yet). Contact the CertMaster with your details and we'll try to associate your new client authentication certificate with your original account.