The only think a user can do directly is an account restore. This restores deleted folders and email messages, but does not restore any of our settings (such as Aliases or Identities).
Adding an alias normally creates an associated Identity. Deleting an alias normally removes the associated Identity. So if you deleted several aliases, those identities should also be now missing. Other people can now capture control of those aliases, so you need to act quickly.
If this happened recently (the past week or so), I think that Fastmail support staff can probably help you. File a support request at:
https://www.fastmail.com/support/
Bill