I believe that message means that your Marketo Synch SFDC user has exceeded the number of failed login attempts and is locked out of Salesforce. This can happen if SFDC has an automatic expiration of passwords after a certain time period. You need to change the password of the Marketo Synch user in SFDC and in Marketo's Salesforce setting section in Admin and then have your SFDC administrator re-enable the SFDC user account.