Good thing you're with FastMail. You can restore a backup through your own settings panel. Depending on how much mail you have, it can take a few minutes or a few hours. Mail recovered! Automatically! The day is saved!
But how did this happen? Concerned someone malicious had access to your account and deleted your mail? Check your connection logs! Enable 2FA! Still not sure how it happened? File a ticket with our customer support team and they can diagnose when and how the delete happened.