Welcome to our forums! It sounds like the sent folder might have been deleted, which can only occur from the customer's end. If that folder is missing, the only way we can attempt to restore the folder and messages is by a hosting restore, but there is no guarantee that the emails will be restored. You will want to reach out to our support team regarding your options.
KayJay - GoDaddy | Community Moderator 24/7 support available at x.co/247support