Handling a Compromised Account
If you believe that your account has been compromised (i.e. your password has been hacked), the first thing you should do is change your password.
Once you have changed your password, it's a good idea to inspect your store to make sure nothing was tampered with. Go to Setup & Tools › Advanced tools › Store logs, then click the Staff Action Log tab. This will display a list of recent user logins, as well as changes made to your store.