Your Salesforce sync is not working because the password for your Salesforce sync user expired. You may see an error in Marketo saying:
"describeSObject failed - The users password has expired, you must call SetPassword before attempting any other API operations."
Similar to this:
To resolve this error, you will have to change both your password and security token in Salesforce and then update these credentials in Marketo under Admin -> Salesforce -> Edit Credentials -> update both your password and security token in the 2 boxes illustrated below then press the Save button:
To permanently fix this issue, you will need to change the password policies in Salesforce so that your password never expires in Salesforce. To do this, in Salesforce, go to Setup -> Administration Setup -> Security Controls -> Password Policies -> User passwords expire in -> Never expires -> Save for your "firstname.lastname@example.org" user in Salesforce:
Please note this is a Salesforce setting and you should consult your Salesforce Admin before implementing this change
Whenever the Salesforce user password expires (it periodically expires for security reason), Marketo stops syncing with the Salesforce account. As a result, any of the leads created in Marketo during this down-time won't be synced to Salesforce.
Marketo will not automatically re-sync. Once you have reset your password and the API calls work once more, you can create a batch campaign that will sync the new leads that have been created in Marketo to Saleforce.