Stop mining on your old account and start mining with a new account.
The important thing is to change the passwords on both your Braiins Pool and email accounts. Also, you should change your password for any other services where you use the same password as in the hacked account.
To prevent this from happening in the future, we strongly recommend that you setup two-factor authentication. And remember: you should never use the same password for multiple services (for security reasons). You should also avoid using short and predictable passwords.