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.
Will Marketo automatically re-sync these leads with Salesforce after I reset my password and the API call starts working again?
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.