To protect your account, Google will sometimes stop outside connections to your mailbox. When this happens Google will simply want you to log back into your account to establish the connection.
If you are getting this error:
Navigate to this link under the Google account in question:
https://accounts.google.com/b/0/DisplayUnlockCaptcha and click "Continue"Then go to Mailstand and try to sync your mailbox again
If you run into any issues, please contact support!