Hello,
Are you still able to sign in to your account right now?
Sign in and secure the account first by changing your password and turning on two step verification in Security settings, then go to Settings, Mail, Rules and remove any unknown rules, also check Forwarding and Connected accounts and remove anything you did not add, then review Sign in activity for suspicious access and sign out all sessions. For recovery, use Deleted Items and Recover items deleted from this folder to restore messages if available, but server level content changes usually cannot be rolled back, so submit a support request through the account help page to check if recovery is possible.