For re-securing your environment: Personal Safety User Guide - Apple Support
For your Apple ID: If you think your Apple ID has been compromised - Apple Support
For your mail account, change the mail server password. Change all of your passwords.
Work through the list of security recommendations in Settings > Passwords > Security Recommendations, too.
If you suspect your iPhone itself to be compromised, factory reset and do not restore a backup.
All that written, not having your email seems like you might simply want to log into the mail provider's mail web portal using your preferred web browser, and see if your mail is still accessible there. This wouldn't be the first time that an account got corrupted, and that corruption rendered mail inaccessible. No hack for these cases, just a benign mess with the local configuration. Removing and re-adding the mail account can usually resolve this case.